Display device

ABSTRACT

By applying an AC pulse to a gate of a transistor which easily deteriorates, a shift in threshold voltage of the transistor is suppressed. However, in a case where amorphous silicon is used for a semiconductor layer of a transistor, the occurrence of a shift in threshold voltage naturally becomes a problem for a transistor which constitutes a part of circuit that generates an AC pulse. A shift in threshold voltage of a transistor which easily deteriorates and a shift in threshold voltage of a turned-on transistor are suppressed by signal input to a gate electrode of the transistor which easily deteriorates through the turned-on transistor. In other words, a structure for applying an AC pulse to a gate electrode of a transistor which easily deteriorates through a transistor to a gate electrode of which a high potential (VDD) is applied, is included.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a divisional of U.S. application Ser. No.11/863,913, filed Sep. 28, 2007, now allowed, which claims the benefitof a foreign priority application filed in Japan as Serial No.2006-269689 on Sep. 29, 2006, both of which are incorporated byreference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a display device which includes acircuit configured using a transistor. The present inventionparticularly relates to a display device using a light emitting element,an electro-optical element such as a liquid crystal element, or the likeas a display medium and to a method for driving the display device.

2. Description of the Related Art

In recent years, with the increase of large-scale display devices suchas liquid crystal televisions, display devices have been activelydeveloped. In particular, a technique for forming a pixel circuit and adriver circuit including a shift register and the like (hereinafter alsoreferred to as an internal circuit) over the same insulating substrateby using transistors formed of a non-crystalline semiconductor(hereinafter also referred to as amorphous silicon) has been activelydeveloped because the technique greatly contributes to reductions inpower consumption and cost. The internal circuit formed over theinsulating substrate is connected to a controller IC or the like(hereinafter also referred to as an external circuit) through an FPC orthe like, and thus its operation is controlled.

Among the aforementioned internal circuits, a shift register usingtransistors formed of a non-crystalline semiconductor (hereinafter alsoreferred to as amorphous silicon transistors) has been devised. FIG.106A shows a structure of a flip-flop included in a conventional shiftregister (Reference 1: Japanese Published Patent Application No.2004-157508). The flip-flop of FIG. 106A includes a transistor 11, atransistor 12, a transistor 13, a transistor 14, a transistor 15, atransistor 16, and a transistor 17, and is connected to a signal line21, a signal line 22, a wiring 23, a signal line 24, a power supply line25, and a power supply line 26. A start signal, a reset signal, a clocksignal, a power supply potential VDD, and a power supply potential VSSare input to the signal line 21, the signal line 22, the signal line 24,the power supply line 25, and the power supply line 26, respectively. Anoperation period of the flip-flop of FIG. 106A is divided into a setperiod, a selection period, a reset period, and a non-selection periodas shown in a timing chart of FIG. 106B, and most of the operationperiod is a non-selection period.

Here, the transistor 12 and the transistor 16 are turned on in anon-selection period. Because amorphous silicon is used forsemiconductor layers of the transistor 12 and the transistor 16,fluctuation in threshold voltage (Vth) is caused due to deterioration orthe like. Specifically, a threshold voltage is increased. In otherwords, a conventional shift register, in which the transistor 12 and thetransistor 16 cannot be turned on due to an increase in thresholdvoltage, cannot supply VSS to a node 41 and the wiring 23 and causesmalfunction.

In order to solve this problem, shift registers which can suppress ashift in threshold voltage of the transistor 12 have been devised inReferences 2, 3, and 4 (Reference 2: Soo Young Yoon et al., “HighlyStable Integrated Gate Driver Circuit using a-Si TFT with Dual Pull-downStructure”, SOCIETY FOR INFORMATION DISPLAY 2005 INTERNATIONAL SYMPOSIUMDIGEST OF TECHNICAL PAPERS, Volume XXXVI, pp. 348-351, Reference 3: BinnKim et al., “a-Si Gate Driver Integration with Time Shared DataDriving”, Proceedings of The 12^(th) International Display Workshops inconjunction with Asia Display 2005, pp. 1073-1076, and Reference 4:Mindoo Chun et al., “Integrated Gate Driver Using Highly Stable a-SiTFT's”, Proceedings of The 12^(th) International Display Workshops inconjunction with Asia Display 2005, pp. 1077-1080). In References 2, 3,and 4, an additional transistor (called a first transistor) is arrangedin parallel with the transistor 12 (called a second transistor), andsignals inverted with respect to each other are input to gate electrodesof the first transistor and the second transistor in a non-selectionperiod; thus, shifts in threshold voltage of the first transistor andthe second transistor are suppressed.

Further, in Reference 5, a shift register, which can suppress a shift inthreshold voltage of the transistor 16 as well as the transistor 12, hasbeen devised (Reference 5: Chun-Ching et al., “Integrated Gate DriverCircuit Using a-Si TFT”, Proceedings of The 12^(th) InternationalDisplay Workshops in conjunction with Asia Display 2005, pp. 1023-1026).In Reference 5, an additional transistor (called a first transistor) isarranged in parallel with the transistor 12 (called a second transistor)and another additional transistor (called a third transistor) isarranged in parallel with the transistor 16 (called a fourthtransistor). In a non-selection period, signals inverted with respect toeach other are input to gate electrodes of the first transistor and thesecond transistor, and signals inverted with respect to each other areinput to gate electrodes of the third transistor and the fourthtransistor; thus, shifts in threshold voltage of the first, second,third, and fourth transistors are suppressed.

Furthermore, in Reference 6, a shift in threshold voltage of thetransistor 12 is suppressed by applying an AC pulse to the gateelectrode of the transistor 12 (Reference 6: Yong Ho Jang et al., “A-SiTFT Integrated Gate Driver with AC-Driven Single Pull-down Structure”,SOCIETY FOR INFORMATION DISPLAY 2006 INTERNATIONAL SYMPOSIUM DIGEST OFTECHNICAL PAPERS, Volume XXXVII, pp. 208-211).

Note that in each of the display devices in References 7 and 8, thenumber of signal lines is reduced to one third by using a shift registerformed using an amorphous silicon transistor as a scan line drivercircuit and inputting a video signal to each of subpixels of R, G, and Bthrough one signal line (Reference 7: Jin Young Choi et al., “A Compactand Cost-efficient TFT-LCD through the Triple-Gate Pixel Structure”,SOCIETY FOR INFORMATION DISPLAY 2006 INTERNATIONAL SYMPOSIUM DIGEST OFTECHNICAL PAPERS, Volume XXXVII, pp. 274-276, and Reference 8: Yong SoonLee et al., “Advanced TFT-LCD Data Line Reduction Method”, SOCIETY FORINFORMATION DISPLAY 2006 INTERNATIONAL SYMPOSIUM DIGEST OF TECHNICALPAPERS, Volume XXXVII, pp. 1083-1086). Thus, in each of the displaydevices in References 7 and 8, the number of connections between adisplay panel and a driver IC is reduced.

SUMMARY OF THE INVENTION

According to the conventional art, by applying an AC pulse to a gate ofa transistor which easily deteriorates, a shift in threshold voltage ofthe transistor is suppressed. However, in a case where amorphous siliconis used for a semiconductor layer of the transistor, the occurrence of ashift in threshold voltage naturally becomes a problem for a transistorincluded in a circuit that generates an AC pulse.

In addition, although a reduction in number of contact points between adisplay panel and a driver IC through a reduction in number of signallines to one third has been proposed (References 7 and 8), a furtherreduction in number of contact points of a driver IC is practicallyneeded.

In other words, objects left unachieved by the conventional art are asfollows: a circuit technology for suppressing a fluctuation in thresholdvoltage of a transistor; a technique for reducing the number of contactpoints of a driver IC mounted on a display panel; a reduction in powerconsumption of a display device; and an increase in size or definitionof a display device.

It is an object of the invention disclosed by this specification toprovide an industrially-useful technique by achieving one or more ofthese objects.

A display device according to the present invention suppresses a shiftin threshold voltage of a transistor which easily deteriorates and ashift in threshold voltage of a transistor in an on state by signalinput to a gate electrode of the transistor which easily deterioratesthrough the transistor in an on state. In other words, the presentinvention includes a structure for applying an AC pulse to a gateelectrode of a transistor which easily deteriorates through a transistor(or an element having a resistance) to a gate electrode of which a highpotential (VDD) is applied.

Switches in this specification can be of various types. An electricalswitch, a mechanical switch, and the like are given as examples. Thatis, any element can be used as long as it can control a current flow,without limitation to a particular element. For example, a transistor(e.g., a bipolar transistor or a MOS transistor), a diode (e.g., a PNdiode, a PIN diode, a Schottky diode, a MIM (Metal Insulator Metal)diode, a MIS (Metal Insulator Semiconductor) diode, or a diode-connectedtransistor), a thyristor, or the like can be used as a switch.Alternatively, a logic circuit combining such elements can be used as aswitch.

In the case of using a transistor as a switch, polarity (a conductivitytype) of the transistor is not particularly limited because it operatesjust as a switch. However, a transistor having polarity with smalleroff-current is preferably used when off-current should be small. Atransistor provided with an LDD region, a transistor with a multi-gatestructure, and the like are given as examples of a transistor withsmaller off-current. When a transistor, which is operated as a switch,operates with a potential of its source terminal close to alow-potential-side power supply (e.g., VSS, GND, or 0 V), an n-channeltransistor is preferably used. On the other hand, when a transistor,which is operated as a switch, operates with a potential of its sourceterminal closer to a potential of a high-potential-side power supply(e.g., VDD), a p-channel transistor is preferably used. This is becausethe absolute value of gate-source voltage can be increased and switchingcharacteristics become favorable when an n-channel transistor operateswith a potential of its source terminal closer to a low-potential-sidepower supply or when a p-channel transistor operates with a potential ofits source terminal closer to a potential of a high-potential-side powersupply. This is also because the transistors hardly conduct a sourcefollower operation, so that reduction in output voltage hardly occurs.

A CMOS switch may be employed as a switch by using both N-channel andp-channel transistors. A CMOS switch can more precisely operate as aswitch because current can flow when either the p-channel transistor orthe n-channel transistor is turned on. For example, voltage can beappropriately output regardless of whether voltage of an input signal ofthe switch is high or low. In addition, since a voltage amplitude valueof a signal for turning on or off the switch can be made small, powerconsumption can be reduced.

When a transistor is employed as a switch, the switch includes an inputterminal (one of a source terminal and a drain terminal), an outputterminal (the other of the source terminal and the drain terminal), anda terminal for controlling electrical conduction (a gate terminal). Onthe other hand, when a diode is employed as a switch, some switches donot have a terminal for controlling electrical conduction. Therefore,the number of wirings for controlling terminals can be more reduced thanthe case of using a transistor, when a diode is used as a switch.

When it is explicitly described in this specification that “A and B areconnected”, the case where A and B are electrically connected, the casewhere A and B are functionally connected, and the case where A and B aredirectly connected are included therein. Here, each of A and B is anobject (e.g., a device, an element, a circuit, a wiring, an electrode, aterminal, a conductive film, or a layer). Accordingly, connections otherthan the connections described in this specification and illustrated inthe drawings are also included in the structures disclosed by thisspecification, without limitations to predetermined connections and theconnections described in this specification and illustrated in thedrawings.

For example, in the case where A and B are electrically connected, oneor more elements which enable electrical connection of A and B (e.g., aswitch, a transistor, a capacitor, an inductor, a resistor, and/or adiode) may be provided between A and B. In addition, in the case where Aand B are functionally connected, one or more circuits which enablefunctional connection of A and B (e.g., a logic circuit such as aninverter, a NAND circuit, or a NOR circuit, a signal converter circuitsuch as a DA converter circuit, an AD converter circuit, or a gammacorrection circuit, a potential level converter circuit such as a powersupply circuit (e.g., a step-up circuit or a step-down circuit) or alevel shifter circuit for changing a potential level of a signal, avoltage source, a current source, a switching circuit, an amplifiercircuit such as a circuit which can increase signal amplitude, theamount of current, or the like (e.g., an operational amplifier, adifferential amplifier circuit, a source follower circuit, or a buffercircuit), a signal generating circuit, a memory circuit, and/or acontrol circuit) may be provided between A and B. Alternatively, in thecase where A and B are directly connected, A and B may be directlyconnected without interposing another element or another circuittherebetween.

When it is explicitly described that “A and B are directly connected”,the case where A and B are directly connected (i.e., the case where Aand B are connected without interposing another element or anothercircuit therebetween) and the case where A and B are electricallyconnected (i.e., the case where A and B are connected with anotherelement or another circuit interposed therebetween) are includedtherein.

When it is explicitly described that “A and B are electricallyconnected”, the case where A and B are electrically connected (i.e., thecase where A and B are connected with another element or another circuitinterposed therebetween), the case where A and B are functionallyconnected (i.e., the case where A and B are functionally connected withanother circuit interposed therebetween), and the case where A and B aredirectly connected (i.e., the case where A and B are connected withoutinterposing another element or another circuit therebetween) areincluded therein. That is, when it is explicitly described that “A and Bare electrically connected”, the description is the same as the casewhere it is explicitly and simply described that “A and B areconnected”.

A display element, a display device which is a device having a displayelement, a light-emitting element, and a light-emitting device which isa device having a light-emitting element can be of various types and caninclude various elements. For example, as a display element, a displaydevice, a light-emitting element, or a light-emitting device, a displaymedium whose contrast, luminance, reflectivity, transmittivity, or thelike changes by an electromagnetic action, such as an EL element (e.g.,an organic EL element, an inorganic EL element, or an EL elementincluding both organic and inorganic materials), an electron-emissiveelement, a liquid crystal element, electronic ink, an electrophoresiselement, a grating light valve (GLV), a plasma display panel (PDP), adigital micromirror device (DMD), a piezoelectric ceramic display, or acarbon nanotube can be employed. Note that display devices using an ELelement include an EL display; display devices using anelectron-emissive element include a field emission display (FED), anSED-type flat panel display (SED: Surface-conduction Electron-emitterDisplay), and the like; display devices using a liquid crystal elementinclude a liquid crystal display (e.g., a transmissive liquid crystaldisplay, a semi-transmissive liquid crystal display, a reflective liquidcrystal display, a direct-view liquid crystal display, or a projectionliquid crystal display); and display devices using electronic ink or anelectrophoresis element include electronic paper.

Transistors in this specification can be of various types withoutlimitations to a particular type. For example, thin film transistors(TFT) including a non-single crystalline semiconductor film typified byamorphous silicon, polycrystalline silicon, microcrystal (also referredto as semi-amorphous) silicon, or the like can be employed. In the caseof using such TFTs, there are various advantages. For example, sinceTFTs can be formed at temperature lower than those using singlecrystalline silicon, the manufacturing cost can be reduced and the sizeof a manufacturing device can be increased. Since the manufacturingdevice can be made larger, the TFTs can be formed using a largesubstrate. Therefore, since a large number of display devices can beformed at the same time, they can be formed at low cost. In addition,because the manufacturing temperature is low, a substrate having lowheat resistance can be used. Thus, transistors can be formed using alight-transmitting substrate. Further, transmission of light in adisplay element can be controlled by using the transistors formed usingthe light-transmitting substrate. Furthermore, a part of a film whichforms a transistor can transmit light because film thickness of thetransistor is thin. Accordingly, an aperture ratio can be improved.

By using a catalyst (e.g., nickel) in the case of formingpolycrystalline silicon, crystallinity can be more improved and atransistor having excellent electric characteristics can be formed.Accordingly, a gate driver circuit (e.g., a scan line driver circuit), asource driver circuit (e.g., a signal line driver circuit), and a signalprocessing circuit (e.g., a signal generation circuit, a gammacorrection circuit, or a DA converter circuit) can be formed over thesame substrate.

By using a catalyst (e.g., nickel) in the case of forming microcrystalsilicon, crystallinity can be more improved and a transistor havingexcellent electric characteristics can be formed. At this time,crystallinity can be improved by performing heat treatment without usinga laser. Accordingly, a gate driver circuit (e.g., a scan line drivercircuit) and a part of a source driver circuit (e.g., an analog switch)can be formed over the same substrate. In addition, in the case of notusing a laser for crystallization, crystallinity unevenness (mura) ofsilicon can be suppressed. Therefore, an image having high image qualitycan be displayed.

Note that polycrystalline silicon and microcrystal silicon can be formedwithout using a catalyst (e.g., nickel).

In addition, a transistor can be formed by using a semiconductorsubstrate, an SOI substrate, or the like. In that case, a MOStransistor, a junction transistor, a bipolar transistor, or the like canbe used as the transistor in this specification. With such a transistor,a transistor with almost no variations in characteristics, sizes,shapes, or the like, with high current supply capacity, and with a smallsize can be formed. By using such a transistor, a circuit which consumesless power can be structured, or higher integration can be achieved.

In addition, a transistor including a compound semiconductor or an oxidesemiconductor such as ZnO, a-InGaZnO, SiGe, GaAs, IZO, ITO (indium tinoxide), or SnO, and a thin film transistor or the like with a thin filmof such a compound semiconductor or an oxide semiconductor can be used.Therefore, manufacturing temperature can be lowered and for example,such a transistor can be formed at room temperature. Accordingly, thetransistor can be formed directly on a substrate having low heatresistance such as a plastic substrate or a film substrate. Note thatsuch a compound semiconductor or an oxide semiconductor can be used fornot only a channel portion of a transistor but also other applications.For example, such a compound semiconductor or an oxide semiconductor canbe used for a resistor, a pixel electrode, or a light-transmittingelectrode. Further, since such an element can be formed at the same timeas the transistor, the cost can be reduced.

Transistors or the like formed by using an inkjet method or a printingmethod can also be used. Accordingly, transistors can be formed at roomtemperature, can be formed at a low vacuum, or can be formed using alarge substrate. In addition, since transistors can be formed withoutusing a mask (a reticle), layout of the transistors can be easilychanged. Further, since it is not necessary to use a resist, thematerial cost is reduced and the number of steps can be reduced.Furthermore, since a film is formed only in a necessary portion, amaterial is not wasted compared with a manufacturing method in whichetching is performed after a film is formed over the entire surface, sothat the cost can be reduced.

Further, transistors or the like including an organic semiconductor or acarbon nanotube can be used. Accordingly, such transistors can be formedusing a bendable or flexible substrate. Therefore, such transistors canresist a shock.

Furthermore, various transistors other than the above-described typescan be used.

Substrates over which transistors are formed can be of various types andare not limited to those of specific types. Examples of substrate overwhich transistors are formed are: a single crystalline substrate; an SOIsubstrate; a glass substrate; a quartz substrate; a plastic substrate; apaper substrate; a cellophane substrate; a stone substrate; a woodsubstrate; a cloth substrate (including a natural fiber (e.g., silk,cotton, or hemp), a synthetic fiber (e.g., nylon, polyurethane, orpolyester), a regenerated fiber (e.g., acetate, cupra, rayon, orregenerated polyester), or the like); a leather substrate; a rubbersubstrate; a stainless-steel substrate; a substrate includingstainless-steel foil; and the like. Alternatively, a skin (e.g., cuticleor corium) or hypodermal tissue of an animal such as a human being canbe used as a substrate. In addition, transistors may be formed using asubstrate, and then, the transistors may be transferred to anothersubstrate. As a substrate to which the transistors are transferred, asingle crystalline substrate, an SOI substrate, a glass substrate, aquartz substrate, a plastic substrate, a paper substrate, a cellophanesubstrate, a stone substrate, a wood substrate, a cloth substrate(including a natural fiber (e.g., silk, cotton, or hemp), a syntheticfiber (e.g., nylon, polyurethane, or polyester), a regenerated fiber(e.g., acetate, cupra, rayon, or regenerated polyester), or the like), aleather substrate, a rubber substrate, a stainless-steel substrate, asubstrate including stainless-steel foil, or the like can be used. Byusing such a substrate, transistors with excellent properties ortransistors which consume less power can be formed, a device with highdurability or high heat resistance can be formed, or reduction in weightcan be achieved.

Transistors can have various structures without limitation to a certainstructure. For example, a multi-gate structure having two or more gateelectrodes may be used. In the multi-gate structure, a plurality oftransistors are connected in series because channel regions areconnected in series. By using the multi-gate structure, off-current canbe reduced or the withstand voltage of transistors can be increased toimprove reliability. Alternatively, drain-source current does notfluctuate very much even if drain-source voltage fluctuates when thetransistor operates in a saturation region, so that voltage-currentcharacteristics with a flat slope can be obtained. By utilizing thevoltage-current characteristics with a flat slope, an ideal currentsource circuit or an active load having an extremely high resistancevalue can be provided. Accordingly, a differential circuit or a currentmirror circuit having excellent properties can be provided.Alternatively, a structure where gate electrodes are formed above andbelow a channel may be used. By using the structure where gateelectrodes are formed above and below the channel, a channel region isenlarged, so that the amount of current flowing therethrough can beincreased. Alternatively, a depletion layer can be easily formed todecrease a subthreshold swing (S value). When the gate electrodes areformed above and below the channel, a structure where a plurality oftransistors are connected in parallel is provided.

Alternatively, a structure where a gate electrode is formed above achannel region, or a structure where a gate electrode is formed below achannel region may be employed. Still alternatively, a staggeredstructure or an inverted staggered structure may be employed; a channelregion may be divided into a plurality of regions; channel regions maybe connected in parallel; or channel regions may be connected in series.In addition, a source electrode or a drain electrode may overlap with achannel region (or part of it). By using the structure where the sourceelectrode or the drain electrode overlaps with the channel region (orpart of it), an unstable operation due to electric charges accumulatedin part of the channel region can be prevented. Further, an LDD regionmay be provided. By providing the LDD region, off-current can be reducedor the withstand voltage of transistors can be increased to improvereliability. Alternatively, by providing the LDD region, drain-sourcecurrent does not fluctuate so much even if drain-source voltagefluctuates when a transistor operates in the saturation region, so thatvoltage-current characteristics with a flat slope can be obtained.

In this specification, one pixel corresponds to the smallest unit of animage. Accordingly, in the case of a full-color display device havingcolor elements of R (Red), G (Green), and B (Blue), one pixel includes adot of a color element of R (Red), a dot of a color element of G(Green), and a dot of a color element of B (Blue). Note that the colorelements are not limited to three colors, and color elements of three ormore colors may be used or a color other than RGB may be used. Forexample, RGBW (W corresponds to white) may be used by adding white. Inaddition, RGB plus one or more colors of yellow, cyan, magenta, emeraldgreen, vermilion, and the like may be used. Further, a color similar toat least one of R, G, and B may be added to RGB. For example, R, G, B1,and B2 may be used. Although both B1 and B2 are blue, they have slightlydifferent frequency. Similarly, R1, R2, G, and B may be used. By usingsuch color elements, display which is closer to the real object can beperformed or power consumption can be reduced. Note that one pixel mayinclude a plurality of dots of color elements of the same color. In thatcase, the plurality of color elements may have different sizes ofregions that contribute to display. In addition, by separatelycontrolling the plurality of dots of color elements of the same color,grayscale may be expressed. This method is referred to as anarea-grayscale method. Alternatively, with the use of the plurality ofdots of color elements of the same color, signals supplied to each ofthe plurality of dots may be slightly varied to widen a viewing angle.That is, potentials of pixel electrodes included in a plurality of colorelements of the same color may be different from each other.Accordingly, voltages applied to liquid crystal molecules are varieddepending on the pixel electrodes. Therefore, the viewing angle can bewidened.

In this specification, one pixel corresponds to one element whosebrightness can be controlled. Therefore, as an example, one pixelcorresponds to one color element and brightness is expressed with theone color element. Accordingly, in the case of a color display devicehaving color elements of R (Red), G (Green), and B (Blue), the minimumunit of an image is formed of three pixels of an R pixel, a G pixel, anda B pixel. Note that the color elements are not limited to three colors,and color elements of three or more colors may be used or a color otherthan RGB may be used. For example, RGBW (W corresponds to white) may beused by adding white. In addition, RGB plus one or more colors ofyellow, cyan, magenta, emerald green, vermilion, and the like can beused. Further, a color similar to at least one of R, G, and B may beadded to RGB. For example, R, G, B1, and B2 may be used. Although bothB1 and B2 are blue, they have slightly different frequency. Similarly,R1, R2, G, and B may be used. By using such color elements, displaywhich is closer to the real object can be performed, or powerconsumption can be reduced. Alternatively, as another example, in thecase of controlling brightness of one color element by using a pluralityof regions, one region may correspond to one pixel. Therefore, as anexample, in the case of performing area ratio grayscale display or thecase of including subpixels, a plurality of regions which controlbrightness are provided for each color element and grayscales areexpressed with all of the regions. In this case, one region whichcontrols brightness may correspond to one pixel. Thus, in this case, onecolor element includes a plurality of pixels. Alternatively, even when aplurality of regions which control brightness are provided in one colorelement, one color element including the plurality of regions maycorrespond to one pixel. Thus, in this case, one color element includesone pixel. Further, when brightness is controlled by a plurality ofregions for each color element, regions which contribute to display havedifferent area dimensions depending on pixels in some cases. Inaddition, in a plurality of regions which control brightness in eachcolor element, signals supplied to each of the plurality of regions maybe slightly varied to widen a viewing angle. That is, potentials ofpixel electrodes included in a plurality of regions provided in eachcolor element may be different from each other. Accordingly, voltagesapplied to liquid crystal molecules are varied depending on the pixelelectrodes. Therefore, the viewing angle can be widened.

When “one pixel (for three colors)” is explicitly described, itcorresponds to the case where three pixels of R, G, and B are consideredas one pixel. Meanwhile, when “one pixel (for one color)” is explicitlydescribed, it corresponds to the case where a plurality of regions areprovided for each color element and collectively considered as onepixel.

In this specification, pixels are provided (arranged) in matrix in somecases. Here, description that pixels are provided (arranged) in matrixincludes the case where the pixels are arranged in a straight line andthe case where the pixels are arranged in a jagged line, in alongitudinal direction or a lateral direction. For example, in the caseof performing full color display with three color elements (e.g., RGB),a case where pixels are arranged in stripes and a case where dots of thethree color elements are arranged in a delta pattern are included.Additionally, a case which dots of the three color elements are providedin Bayer arrangement is also included. Note that the color elements arenot limited to three colors, and more than three color elements may beemployed. RGBW (W corresponds to white), RGB plus one or more of yellow,cyan, magenta, and the like, or the like is given as an example.Further, the sizes of display regions may be different betweenrespective dots of color elements. Thus, power consumption can bereduced, or the life of a display element can be prolonged.

In this specification, an active matrix method in which an activeelement is included in a pixel or a passive matrix method in which anactive element is not included in a pixel can be used.

In the active matrix method, as an active element (a non-linearelement), not only a transistor but also various active elements(non-linear elements) can be used. For example, a MIM (Metal InsulatorMetal), a TFD (Thin Film Diode), or the like can also be used. Sincesuch an element needs a smaller number of manufacturing steps, theelement can be manufactured at low cost, or a yield can be improved.Further, since the size of such an element is small, an aperture ratiocan be improved, so that power consumption can be reduced and higherluminance can be achieved.

As a method other than the active matrix method, the passive matrixmethod in which an active element (a non-linear element) is not used canalso be used. Since an active element (a non-linear element) is notused, the number of manufacturing steps is smaller, so that the elementcan be manufactured at low cost, or the yield can be improved. Further,since an active element (a non-linear element) is not used, the apertureratio can be improved, so that power consumption can be reduced and highluminance can be achieved.

Note that a transistor is an element having at least three terminals ofa gate, a drain, and a source. The transistor has a channel regionbetween a drain region and a source region, and current can flow throughthe drain region, the channel region, and the source region. Here, sincethe source and the drain of the transistor may change depending on thestructure, the operating condition, etc., of the transistor, it isdifficult to define which is a source or a drain. Therefore, in thisspecification, a region functioning as a source and a drain is notcalled the source or the drain in some cases. In such a case, forexample, one of the source and the drain may be described as a firstelectrode and the other thereof may be described as a second electrode.

A transistor may be an element having at least three terminals of abase, an emitter, and a collector. In this case also, one of the emitterand the collector may be similarly called a first terminal and the otherterminal may be called a second terminal.

A gate corresponds to the whole or part of a gate electrode and a gatewiring (also called a gate line, a gate signal line, a scan line, a scansignal line, or the like). A gate electrode corresponds to part of aconductive film which overlaps with a semiconductor which forms achannel region with a gate insulating film interposed therebetween. Notethat part of the gate electrode overlaps with an LDD (Lightly DopedDrain) region, or the source region and the drain region with the gateinsulating film interposed therebetween in some cases. A gate wiringcorresponds to a wiring for connection between gate electrodes oftransistors, or a wiring for connection between a gate electrode andanother wiring.

Note that there is a portion (a region, a conductive film, a wiring, orthe like) which acts as both a gate electrode and a gate wiring. Such aportion (a region, a conductive film, a wiring, or the like) may becalled either a gate electrode or a gate wiring. That is, there is aregion where a gate electrode and a gate wiring cannot be clearlydistinguished from each other. For example, in the case where a channelregion overlaps with part of an extended gate wiring, the overlappedportion (region, conductive film, wiring, or the like) functions as botha gate wiring and a gate electrode. Accordingly, such a portion (aregion, a conductive film, a wiring, or the like) may be called either agate electrode or a gate wiring.

In addition, a portion (a region, a conductive film, a wiring, or thelike) which is formed of the same material as a gate electrode and whichforms the same island as the gate electrode to be connected to the gateelectrode may also be called a gate electrode. Similarly, a portion (aregion, a conductive film, a wiring, or the like) which is formed of thesame material as a gate wiring and which forms the same island as thegate wiring to be connected to the gate wiring may also be called a gatewiring. In a strict sense, such a portion (a region, a conductive film,a wiring, or the like) does not overlap with a channel region and doesnot have a function of connecting a gate electrode to another gateelectrode in some cases. However, there is a portion (a region, aconductive film, a wiring, or the like) which is formed of the samematerial as a gate electrode or a gate wiring and which forms the sameisland as the gate electrode or the gate wiring to be connected to thegate electrode or the gate wiring. Such a portion (a region, aconductive film, a wiring, or the like) may also be called either a gateelectrode or a gate wiring.

In a multi-gate transistor, for example, a gate electrode is oftenconnected to another gate electrode by using a conductive film which isformed of the same material as the gate electrodes. Since such a portion(a region, a conductive film, a wiring, or the like) is a portion (aregion, a conductive film, a wiring, or the like) for connecting thegate electrode to another gate electrode, it may be called a gatewiring, but it may also be called a gate electrode because a multi-gatetransistor can be considered as one transistor. That is, a portion (aregion, a conductive film, a wiring, or the like) which is formed of thesame material as a gate electrode or a gate wiring and which forms thesame island as the gate electrode or the gate wiring to be connected tothe gate electrode or the gate wiring may be called either a gateelectrode or a gate wiring. In addition, for example, part of aconductive film which connects a gate electrode and a gate wiring andwhich is formed from a different material from the gate electrode andthe gate wiring may also be called either a gate electrode or a gatewiring.

A gate terminal corresponds to part of a portion (a region, a conductivefilm, a wiring, or the like) of a gate electrode or a portion (a region,a conductive film, a wiring, or the like) which is electricallyconnected to the gate electrode.

When a wiring is called a gate wiring, a gate line, a gate signal line,a scan line, a scan signal line, or the like, there is a case in which agate of a transistor is not connected to a wiring. In this case, thegate wiring, the gate line, the gate signal line, the scan line, or thescan signal line corresponds to a wiring formed in the same layer as thegate of the transistor, a wiring formed of the same material of the gateof the transistor, or a wiring formed at the same time as the gate ofthe transistor in some cases. Examples are: a wiring for storagecapacitor; a power supply line; a reference potential supply line; andthe like.

A source corresponds to the whole or part of a source region, a sourceelectrode, and a source wiring (also called a source line, a sourcesignal line, a data line, a data signal line, or the like). A sourceregion corresponds to a semiconductor region containing a large amountof p-type impurities (e.g., boron or gallium) or n-type impurities(e.g., phosphorus or arsenic). Accordingly, a region containing a smallamount of p-type impurities or n-type impurities, namely, an LDD(Lightly Doped Drain) region is not included in the source region. Asource electrode is part of a conductive layer which is formed of amaterial different from that of a source region and electricallyconnected to the source region. Note that there is a case where a sourceelectrode and a source region are collectively called a sourceelectrode. A source wiring is a wiring for connection between sourceelectrodes of transistors, or a wiring for connection between a sourceelectrode and another wiring.

However, there is a portion (a region, a conductive film, a wiring, orthe like) functioning as both a source electrode and a source wiring.Such a portion (a region, a conductive film, a wiring, or the like) maybe called either a source electrode or a source wiring. That is, thereis a region where a source electrode and a source wiring cannot beclearly distinguished from each other. For example, in a case where asource region overlaps with part of an extended source wiring, theoverlapped portion (region, conductive film, wiring, or the like)functions as both a source wiring and a source electrode. Accordingly,such a portion (a region, a conductive film, a wiring, or the like) maybe called either a source electrode or a source wiring.

In addition, a portion (a region, a conductive film, a wiring, or thelike) which is formed of the same material as a source electrode andwhich forms the same island as the source electrode to be connected tothe source electrode, or a portion (a region, a conductive film, awiring, or the like) which connects a source electrode and anothersource electrode may also be called a source electrode. Further, aportion which overlaps with a source region may be called a sourceelectrode. Similarly, a portion (a region, a conductive film, a wiring,or the like) which is formed of the same material as a source wiring andwhich forms the same island as the source wiring to be connected to thesource wiring may also be called a source wiring. In a strict sense,such a portion (a region, a conductive film, a wiring, or the like) doesnot have a function of connecting a source electrode to another sourceelectrode in some cases. However, there is a portion (a region, aconductive film, a wiring, or the like) which is formed of the samematerial as a source electrode or a source wiring and is connected tothe source electrode or the source wiring. Thus, such a portion (aregion, a conductive film, a wiring, or the like) may also be calledeither a source electrode or a source wiring.

In addition, for example, part of a conductive film which connects asource electrode and a source wiring and is formed of a materialdifferent from that of the source electrode or the source wiring may becalled either a source electrode or a source wiring.

A source terminal corresponds to part of a source region, a sourceelectrode, or a portion (a region, a conductive film, a wiring, or thelike) which is electrically connected to the source electrode.

When a wiring is called a source wiring, a source line, a source signalline, a data line, a data signal line, or the like, there is a case inwhich a source (a drain) of a transistor is not connected to a wiring.In this case, the source wiring, the source line, the source signalline, the data line, or the data signal line corresponds to a wiringformed in the same layer as the source (the drain) of the transistor, awiring formed of the same material of the source (the drain) of thetransistor, or a wiring formed at the same time as the source (thedrain) of the transistor in some cases. Examples are: a wiring forstorage capacitor; a power supply line; a reference potential supplyline; and the like.

The same applies to a drain as to the source.

A semiconductor device corresponds to a device having a circuitincluding a semiconductor element (e.g., a transistor, a diode, orthyristor). The semiconductor device may be general devices that canfunction by utilizing semiconductor characteristics.

A display element corresponds to an optical modulation element, a liquidcrystal element, a light-emitting element, an EL element (an organic ELelement, an inorganic EL element, or an EL element including organic andinorganic materials), an electron-emissive element, an electrophoresiselement, a discharging element, a light-reflecting element, a lightdiffraction element, a digital micromirror device (DMD), or the like.Note that the present invention is not limited to these examples.

A display device corresponds to a device having a display element. Notethat a display device may correspond to a main body of a display panelwhere a plurality of pixels each including a display element and aperipheral driver circuit for driving these pixels are formed over thesame substrate. In addition, a display device may also include aperipheral driver circuit provided over a substrate by wire bonding orbump bonding, namely, an IC chip connected by chip on glass (COG) or anIC chip connected by TAB or the like. Further, a display device mayinclude a flexible printed circuit (FPC) to which an IC chip, aresistor, a capacitor, an inductor, a transistor, or the like isattached. Note that a display device includes a printed wiring board(PWB) which is connected through a flexible printed circuit (FPC) and towhich an IC chip, a resistor, a capacitor, an inductor, a transistor, orthe like is attached. A display device may also include an optical sheetsuch as a polarizing plate or a retardation plate. A display device mayalso include a lighting device, a housing, an audio input and outputdevice, a light sensor, and the like. Here, a lighting device such as abacklight unit may include a light guide plate, a prism sheet, adiffusion sheet, a reflective sheet, a light source (e.g., an LED or acold cathode tube), a cooling device (e.g., a water cooling device or anair cooling device), or the like.

A lighting device corresponds to a device having a backlight unit, alight guide plate, a prism sheet, a diffusion sheet, a reflective sheet,or a light source (e.g., an LED, a cold cathode tube, or a hot cathodetube), a cooling device, or the like.

A light-emitting device corresponds to a device having a light-emittingelement or the like.

A reflective device corresponds to a device having a light-reflectingelement, a light-diffraction element, a light-reflecting electrode, orthe like.

A liquid crystal display device corresponds to a display deviceincluding a liquid crystal element. Liquid crystal display devicesinclude a direct-view liquid crystal display, a projection liquidcrystal display, a transmissive liquid crystal display, asemi-transmissive liquid crystal display, a reflective liquid crystaldisplay, and the like.

A driving device corresponds to a device having a semiconductor element,an electric circuit, an electronic circuit, and/or the like. Forexample, a transistor which controls input of a signal from a sourcesignal line to a pixel (also called a selection transistor, a switchingtransistor, or the like), a transistor which supplies voltage or currentto a pixel electrode, a transistor which supplies voltage or current toa light-emitting element, and the like are examples of the drivingdevice. A circuit which supplies a signal to a gate signal line (alsocalled a gate driver, a gate line driver circuit, or the like), acircuit which supplies a signal to a source signal line (also called asource driver, a source line driver circuit, or the like) are alsoexamples of the driving device.

A display device, a semiconductor device, a lighting device, a coolingdevice, a light-emitting device, a reflective device, a driving device,and the like are provided together in some cases. For example, a displaydevice includes a semiconductor device and a light-emitting device insome cases, or a semiconductor device includes a display device and adriving device in some cases.

When “B is formed on A” or “B is formed over A” is explicitly describedin this specification, it does not necessarily mean that B is formed indirect contact with A. The description includes a case where A and B arenot in direct contact with each other, i.e., a case where another objectis interposed between A and B. Here, each of A and B corresponds to anobject (e.g., a device, an element, a circuit, a wiring, an electrode, aterminal, a conductive film, or a layer).

Accordingly, for example, when “a layer B is formed on (or over) a layerA” is explicitly described, it includes both a case where the layer B isformed in direct contact with the layer A, and a case where anotherlayer (e.g., a layer C or a layer D) is formed in direct contact withthe layer A, and the layer B is formed in direct contact with the layerC or D. Note that another layer (e.g., a layer C or a layer D) may be asingle layer or a plurality of layers.

Similarly, when “B is formed above (or over) A” is explicitly described,it does not necessarily mean that B is formed in direct contact with A,and another object may be interposed between A and B. Accordingly, forexample, when “a layer B is formed above a layer A” is explicitlydescribed, it includes both a case where the layer B is formed in directcontact with the layer A, and a case where another layer (e.g., a layerC or a layer D) is formed in direct contact with the layer A, and thelayer B is formed in direct contact with the layer C or D. Note thatanother layer (e.g., a layer C or a layer D) may be a single layer or aplurality of layers.

When it is explicitly described that B is formed in direct contact withA, it includes the case where B is formed in direct contact with A, butnot the case where another object is interposed between A and B.

The same applies to a case where “B is formed below or under A” isexplicitly described.

Characteristic deterioration of all transistors included in a shiftregister can be suppressed. Therefore, the malfunction of asemiconductor device employing the shift register, such as a liquidcrystal display device, can be suppressed.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a diagram illustrating a structure of a flip-flop described inEmbodiment Mode 1.

FIG. 2 is a timing chart illustrating an operation of the flip-flopshown in FIG. 1.

FIGS. 3A to 3E are diagrams illustrating operations of the flip-flopshown in FIG. 1.

FIGS. 4A to 4D are diagrams illustrating structures of flip-flopsdescribed in Embodiment Mode 1.

FIGS. 5A to 5D are diagrams illustrating structures of flip-flopsdescribed in Embodiment Mode 1.

FIG. 6 is a timing chart illustrating an operation of a flip-flopdescribed in Embodiment Mode 1.

FIGS. 7A to 7C are diagrams illustrating structures of flip-flopsdescribed in Embodiment Mode 1.

FIG. 8 is a diagram illustrating a structure of a display devicedescribed in Embodiment Mode 1.

FIG. 9 is a timing chart illustrating a write operation of the displaydevice shown in FIG. 8.

FIG. 10 is a diagram illustrating a structure of a shift registerdescribed in Embodiment Mode 1.

FIG. 11 is a timing chart illustrating an operation of the shiftregister shown in FIG. 10.

FIG. 12 is a timing chart illustrating an operation of the shiftregister shown in FIG. 10.

FIG. 13 is a diagram illustrating a structure of a shift registerdescribed in Embodiment Mode 1.

FIG. 14 is a diagram illustrating a structure of a shift registerdescribed in Embodiment Mode 1.

FIG. 15 is a diagram illustrating a structure of a shift registerdescribed in Embodiment Mode 1.

FIG. 16 is a diagram illustrating a structure of a display devicedescribed in Embodiment Mode 2.

FIG. 17 is a diagram illustrating a structure of a shift registerdescribed in Embodiment Mode 1.

FIG. 18 is a diagram illustrating a structure of a display devicedescribed in Embodiment Mode 1.

FIG. 19 is a timing chart illustrating a write operation of the displaydevice shown in FIG. 18.

FIG. 20 is a diagram illustrating a structure of a display devicedescribed in Embodiment Mode 1.

FIGS. 21A to 21C are diagrams illustrating structures of flip-flopsdescribed in Embodiment Mode 1.

FIG. 22 is a diagram illustrating a structure of a display devicedescribed in Embodiment Mode 2.

FIG. 23 is a diagram illustrating a structure of a flip-flop describedin Embodiment Mode 4.

FIG. 24 is a timing chart illustrating an operation of the flip-flopshown in FIG. 23.

FIG. 25 is a top view of the flip-flop shown in FIG. 1.

FIGS. 26A to 26C are diagrams illustrating structures of a buffer shownin FIG. 13.

FIG. 27 is a diagram illustrating a structure of a flip-flop describedin Embodiment Mode 3.

FIG. 28 is a timing chart illustrating an operation of the flip-flopshown in FIG. 27.

FIG. 29 is a diagram illustrating a structure of a shift registerdescribed in Embodiment Mode 3.

FIG. 30 is a timing chart illustrating an operation of the shiftregister shown in FIG. 29.

FIG. 31 is a timing chart illustrating an operation of a flip-flopdescribed in Embodiment Mode 2.

FIG. 32 is a timing chart illustrating an operation of a flip-flopdescribed in Embodiment Mode 2.

FIG. 33 is a diagram illustrating a structure of a shift registerdescribed in Embodiment Mode 2.

FIG. 34 is a diagram illustrating a structure of a shift registerdescribed in Embodiment Mode 2.

FIG. 35 is a timing chart illustrating an operation of the shiftregister shown in FIG. 33.

FIG. 36 is a timing chart illustrating an operation of the shiftregister shown in FIG. 33.

FIG. 37 is a diagram illustrating a structure of a signal line drivercircuit described in Embodiment Mode 5.

FIG. 38 is a timing chart illustrating an operation of the signal linedriver circuit shown in FIG. 37.

FIG. 39 is a diagram illustrating a structure of a signal line drivercircuit described in Embodiment Mode 5.

FIG. 40 is a timing chart illustrating an operation of the signal linedriver circuit shown in FIG. 39.

FIG. 41 is a diagram illustrating a structure of a signal line drivercircuit described in Embodiment Mode 5.

FIGS. 42A to 42C are diagrams illustrating structures of protectivediodes described in Embodiment Mode 6.

FIGS. 43A and 43B are diagrams illustrating structures of protectivediodes described in Embodiment Mode 6.

FIGS. 44A to 44C are diagrams illustrating structures of protectivediodes described in Embodiment Mode 6.

FIGS. 45A to 45C are diagrams illustrating structures of a displaydevice described in Embodiment Mode 7.

FIGS. 46A to 46G are diagrams illustrating a process for manufacturing asemiconductor device according to the present invention.

FIG. 47 is a diagram illustrating a structure of a semiconductor deviceaccording to the present invention.

FIG. 48 is a diagram illustrating a structure of a semiconductor deviceaccording to the present invention.

FIG. 49 is a diagram illustrating a structure of a semiconductor deviceaccording to the present invention.

FIG. 50 is a diagram illustrating a structure of a semiconductor deviceaccording to the present invention.

FIGS. 51A to 51C are diagrams illustrating a method for driving asemiconductor device according to the present invention.

FIGS. 52A to 52C are diagrams illustrating a method for driving asemiconductor device according to the present invention.

FIGS. 53A to 53C are diagrams illustrating structures of display devicesof/with/using a semiconductor device according to the present invention.

FIGS. 54A and 54B are diagrams illustrating peripheral circuitstructures of a semiconductor device according to the present invention.

FIG. 55 is a diagram illustrating peripheral components of asemiconductor device according to the present invention.

FIGS. 56A to 56D are diagrams illustrating peripheral components of asemiconductor device according to the present invention.

FIG. 57 is a diagram illustrating a peripheral component of asemiconductor device according to the present invention.

FIGS. 58A to 58C are diagrams illustrating peripheral circuit structuresof a semiconductor device according to the present invention.

FIG. 59 is a diagram illustrating peripheral components of asemiconductor device according to the present invention.

FIGS. 60A and 60B are diagrams illustrating a panel circuit structure ofa semiconductor device according to the present invention.

FIG. 61 is a diagram illustrating a panel circuit structure of asemiconductor device according to the present invention.

FIG. 62 is a diagram illustrating a panel circuit structure of asemiconductor device according to the present invention.

FIGS. 63A and 63B are cross-sectional views of display elements of asemiconductor device according to the present invention.

FIGS. 64A to 64D are cross-sectional views of display elements of asemiconductor device according to the present invention.

FIGS. 65A to 65D are cross-sectional views of display elements of asemiconductor device according to the present invention.

FIGS. 66A to 66D are cross-sectional views of display elements of asemiconductor device according to the present invention.

FIG. 67 is a cross-sectional view of a pixel of a semiconductor deviceaccording to the present invention.

FIGS. 68A and 68B are cross-sectional views of pixels of a semiconductordevice according to the present invention.

FIGS. 69A and 69B are cross-sectional views of pixels of a semiconductordevice according to the present invention.

FIG. 70 is a pixel layout example of a semiconductor device according tothe present invention.

FIGS. 71A and 71B are pixel layout examples of a semiconductor deviceaccording to the present invention.

FIGS. 72A and 72B are pixel layout examples of a semiconductor deviceaccording to the present invention.

FIGS. 73A and 73B are diagrams illustrating a method for driving asemiconductor device according to the present invention.

FIGS. 74A and 74B are diagrams illustrating a method for driving asemiconductor device according to the present invention.

FIG. 75 is a diagram illustrating a structure of a pixel of asemiconductor device according to the present invention.

FIG. 76 is a diagram illustrating a structure of a pixel of asemiconductor device according to the present invention.

FIG. 77 is a diagram illustrating a structure of a pixel of asemiconductor device according to the present invention.

FIGS. 78A and 78B are a pixel layout example and a cross-sectional viewof a semiconductor device according to the present invention.

FIGS. 79A to 79E are cross-sectional views of display elements of asemiconductor device according to the present invention.

FIGS. 80A to 80C are cross-sectional views of display elements of asemiconductor device according to the present invention.

FIGS. 81A to 81C are cross-sectional views of display elements of asemiconductor device according to the present invention.

FIGS. 82A and 82B are diagrams illustrating a structure of asemiconductor device according to the present invention.

FIG. 83 is a diagram illustrating a structure of a semiconductor deviceaccording to the present invention.

FIG. 84 is a diagram illustrating a structure of a semiconductor deviceaccording to the present invention.

FIG. 85 is a diagram illustrating a structure of a semiconductor deviceaccording to the present invention.

FIGS. 86A to 86C are diagrams illustrating structures of a semiconductordevice according to the present invention.

FIG. 87 is a diagram illustrating a structure of a semiconductor deviceaccording to the present invention.

FIGS. 88A to 88E are diagrams illustrating a method for manufacturing asemiconductor device according to the present invention.

FIGS. 89A and 89B are diagrams illustrating a method for driving asemiconductor device according to the present invention.

FIGS. 90A to 90C are diagrams illustrating a method for driving asemiconductor device according to the present invention.

FIGS. 91A and 91B are diagrams illustrating a method for driving asemiconductor device according to the present invention.

FIG. 92 is a diagram illustrating a structure of a semiconductor deviceaccording to the present invention.

FIGS. 93A and 93B are diagrams illustrating electronic devices using asemiconductor device according to the present invention.

FIG. 94 is a diagram illustrating a structure of a semiconductor deviceaccording to the present invention.

FIGS. 95A to 95C are diagrams illustrating electronic devices using asemiconductor device according to the present invention.

FIG. 96 is a diagram illustrating an electronic device using asemiconductor device according to the present invention.

FIG. 97 is a diagram illustrating an electronic device using asemiconductor device according to the present invention.

FIG. 98 is a diagram illustrating an electronic device using asemiconductor device according to the present invention.

FIG. 99 is a diagram illustrating an electronic device using asemiconductor device according to the present invention.

FIGS. 100A and 100B are diagrams illustrating electronic devices using asemiconductor device according to the present invention.

FIGS. 101A and 101B are diagrams illustrating electronic devices using asemiconductor device according to the present invention.

FIGS. 102A to 102C are diagrams illustrating electronic devices using asemiconductor device according to the present invention.

FIGS. 103A and 103B are diagrams illustrating electronic devices using asemiconductor device according to the present invention.

FIG. 104 is a diagram illustrating an electronic device using asemiconductor device according to the present invention.

FIGS. 105A to 105D are diagrams illustrating structures of a buffershown in FIG. 13.

FIGS. 106A and 106B are diagrams illustrating a structure and a timingchart of conventional art.

DETAILED DESCRIPTION OF THE INVENTION

Embodiment modes of the present invention will be hereinafter describedwith reference to the drawings. Note that the present invention can becarried out in many different modes, and it is easily understood bythose skilled in the art that the mode and the detail of the inventioncan be variously changed without departing from the spirit and the scopethereof. Therefore, the present invention is not interpreted as beinglimited to the description of embodiment modes.

Embodiment Mode 1

This embodiment mode describes structures and driving methods of aflip-flop, a driver circuit including the flip-flop, and a displaydevice including the driver circuit.

A basic structure of a flip-flop of this embodiment mode is describedwith reference to FIG. 1. A flip-flop shown in FIG. 1 includes a firsttransistor 101, a second transistor 102, a third transistor 103, afourth transistor 104, a fifth transistor 105, a sixth transistor 106,and a seventh transistor 107. In this embodiment mode, each of the firsttransistor 101, the second transistor 102, the third transistor 103, thefourth transistor 104, the fifth transistor 105, the sixth transistor106, and the seventh transistor 107 is an n-channel transistor andbecomes conductive when a gate-source voltage (Vgs) exceeds a thresholdvoltage (Vth).

The connection relationship of the flip-flop of FIG. 1 is described. Afirst electrode (one of a source electrode and a drain electrode) of thefirst transistor 101 is connected to a fifth wiring 125, and a secondelectrode (the other of the source electrode and the drain electrode) ofthe first transistor 101 is connected to a third wiring 123. A firstelectrode of the second transistor 102 is connected to a fourth wiring124, and a second electrode of the second transistor 102 is connected tothe third wiring 123. A first electrode of the third transistor 103 isconnected to a sixth wiring 126; a second electrode of the thirdtransistor 103 is connected to a gate electrode of the second transistor102; and a gate electrode of the third transistor 103 is connected to aseventh wiring 127. A first electrode of the fourth transistor 104 isconnected to a ninth wiring 129; a second electrode of the fourthtransistor 104 is connected to the gate electrode of the secondtransistor 102; and a gate electrode of the fourth transistor 104 isconnected to a gate electrode of the first transistor 101. A firstelectrode of the fifth transistor 105 is connected to an eighth wiring128; a second electrode of the fifth transistor 105 is connected to thegate electrode of the first transistor 101; and a gate electrode of thefifth transistor 105 is connected to a first wiring 121. A firstelectrode of the sixth transistor 106 is connected to a tenth wiring130; a second electrode of the sixth transistor 106 is connected to thegate electrode of the first transistor 101; and a gate electrode of thesixth transistor 106 is connected to the gate electrode of the secondtransistor 102. A first electrode of the seventh transistor 107 isconnected to an eleventh wiring 131; a second electrode of the seventhtransistor 107 is connected to the gate electrode of first transistor101; and a gate electrode of the seventh transistor 107 is connected toa second wiring 122.

Note that a connection portion of the gate electrode of the firsttransistor 101, the gate electrode of the fourth transistor 104, thesecond electrode of the fifth transistor 105, the second electrode ofthe sixth transistor 106, and the second electrode of the seventhtransistor 107 is referred to as a node 141. A connection portion of thegate electrode of the second transistor 102, the second electrode of thethird transistor 103, the second electrode of the fourth transistor 104,and the gate electrode of the sixth transistor 106 is referred to as anode 142.

The fourth wiring 124, the ninth wiring 129, the tenth wiring 130, andthe eleventh wiring 131 may be connected to each other or may be asingle wiring. The seventh wiring 127 and the eighth wiring 128 may beconnected to each other or may be a single wiring.

The first wiring 121, the second wiring 122, the third wiring 123, thefifth wiring 125, and the sixth wiring 126 may be referred to as a firstsignal line, a second signal line, a third signal line, a fourth signalline, and a fifth signal line, respectively. The fourth wiring 124, theseventh wiring 127, the eighth wiring 128, the ninth wiring 129, thetenth wiring 130, and the eleventh wiring 131 may be referred to as afirst power supply line, a second power supply line, a third powersupply line, a fourth power supply line, a fifth power supply line, anda sixth power supply line, respectively.

The seventh wiring 127 and the eighth wiring 128 are each supplied witha potential V1, and the fourth wiring 124, the ninth wiring 129, thetenth wiring 130, and the eleventh wiring 131 are each supplied with apotential V2. The relationship of V1>V2 is satisfied.

Signals are each input to the first wiring 121, the second wiring 122,the fifth wiring 125, and the sixth wiring 126. The signal input to thefirst wiring 121 is a start signal; the signal input to the secondwiring 122 is a reset signal; the signal input to the fifth wiring 125is a first clock signal; and the signal input to the sixth wiring 126 isa second clock signal. Further, each of the signals input to the firstwiring 121, the second wiring 122, the fifth wiring 125, and the sixthwiring 126 is a digital signal including an H signal with a potential ofV1 (hereinafter also referred to as an H level) and an L signal with apotential of V2 (hereinafter also referred to as an L level).

Various signals, potentials, or currents may be input to the firstwiring 121, the second wiring 122, and the fourth to eleventh wirings124 to 131.

Through the third wiring 123, a signal is output. The signal outputthrough the third wiring 123 is an output signal of a flip-flop of eachstage and is also a start signal of a flip-flop of the next stage(hereinafter also referred to as a transfer signal). The signal outputthrough the third wiring 123 is a digital signal including an H signalwith a potential of V1 (hereinafter also referred to as an H level) andan L signal with a potential of V2 (hereinafter also referred to as an Llevel).

Next, an operation of the flip-flop shown in FIG. 1 is described withreference to a timing chart of FIG. 2, and FIGS. 3A to 3E. In addition,the timing chart of FIG. 2 is described with an operation period dividedinto a selection period and a non-selection period. Further, the timingchart is described with the non-selection period divided into a firstnon-selection period, a second non-selection period, a set period, and areset period. Furthermore, during an operation period in thenon-selection period except for the set period and the reset period, thefirst non-selection period and the second non-selection period aresequentially repeated.

Note that in FIG. 2, a signal 221, a signal 225, a signal 226, apotential 241, a potential 242, a signal 222, and a signal 223 refer tothe signal input to the first wiring 121, the signal input to the fifthwiring 125, the signal input to the sixth wiring 126, a potential of thenode 141, a potential of the node 142, the signal input to the secondwiring 122, and the signal output through the third wiring 123,respectively.

First, in the set period shown as period A in FIG. 2 and shown in FIG.3A, the fifth transistor 105 is turned on because the signal 221 is atthe H level, and the seventh transistor 107 is turned off because thesignal 222 is at the L level. The potential of the node 141 (thepotential 241) at this time is V1−Vth(105) (Vth(105): a thresholdvoltage of the fifth transistor 105) because the second electrode of thefifth transistor 105 serves as a source electrode and the potential ofthe node 141 is a value obtained by subtracting the threshold voltage ofthe fifth transistor 105 from the potential of the eighth wiring 128.Thus, the first transistor 101 and the fourth transistor 104 are turnedon, and the fifth transistor 105 is turned off. The potential of thenode 142 (the potential 242) at this time is determined by a resistanceratio of the third transistor 103 and the fourth transistor 104 (L/W andan application voltage) and is V2+β (β: a given positive number). Therelationships of β<Vth(102) (Vth(102): a threshold voltage of the secondtransistor 102) and β<Vth(106) (Vth(106): a threshold voltage of thesixth transistor 106) are satisfied. In other words, a potentialdifference (V1−V2) between the potential of the ninth wiring 129 (V2)and the potential of the sixth wiring 126 (V1) is divided by the thirdtransistor 103 and the fourth transistor 104. Accordingly, the secondtransistor 102 and the sixth transistor 106 are turned off. Thus, in theset period, the third wiring 123 is electrically connected to the fifthwiring 125 to which the L signal is input, so that the potential of thethird wiring 123 becomes V2. Accordingly, the L signal is output throughthe third wiring 123. Further, the node 141 is in a floating state withthe potential maintained at V1−Vth(105).

In the selection period shown as period B in FIG. 2 and shown in FIG.3B, the fifth transistor 105 is turned off because the signal 221 is atthe L level, and the seventh transistor 107 remains off because thesignal 222 remains at the L level. The potential of the node 141 at thetime is maintained at V1−Vth(105). Thus, the first transistor 101 andthe fourth transistor 104 remain on. The potential of the node 142 atthis time is V2 because the sixth wiring 126 is at the L level. Thus,the second transistor 102 and the sixth transistor 106 remain off. Here,the H signal is input to the fifth wiring 125, so that the potential ofthe third wiring 123 starts to increase. The potential of the node 141at this time increases by a bootstrap operation from V1−Vth(105) toV1+Vth(101)+α (Vth(101): a threshold voltage of the first transistor101, a: a given positive number). Accordingly, the potential of thethird wiring 123 becomes equal to that of the fifth wiring 125, which isV1. Note that the bootstrap operation is performed by capacitivecoupling of a parasitic capacitance between the gate electrode and thesecond electrode of the first transistor 101. Thus, in the selectionperiod, the third wiring 123 is electrically connected to the fifthwiring 125 to which the H signal is input, so that the potential of thethird wiring 123 is V1. Accordingly, the H signal is output through thethird wiring 123.

In the reset period shown as period C in FIG. 2 and shown in FIG. 3C,the fifth transistor 105 remains off because the signal 221 remains atthe L level, and the seventh transistor 107 is turned on because thesignal 222 is at the H level. The potential of the node 141 at this timeis V2 because the potential of the eleventh wiring 131 (V2) is suppliedthrough the seventh transistor 107. Thus, the first transistor 101 andthe fourth transistor 104 are turned off. The potential of the node 142at this time is V1−Vth(103) (Vth(103): a threshold voltage of the thirdtransistor 103) because the second electrode of the third transistor 103serves as a source electrode and the potential of the node 142 is avalue obtained by subtracting the threshold voltage of the thirdtransistor 103 from the potential of the sixth wiring 126 (V1).Accordingly, the second transistor 102 and the sixth transistor 106 areturned on. Thus, in the reset period, the third wiring 123 iselectrically connected to the fourth wiring 124 to which V2 is supplied,so that the potential of the third wiring 123 is V2. Thus, the L signalis output through the third wiring 123.

In the first non-selection period shown as period D in FIG. 2 and shownin FIG. 3D, the fifth transistor 105 remains off because the signal 221remains at the L level, and the seventh transistor 107 is turned offbecause the signal 222 is at the L level. The potential of the node 142at this time is V2 because the L signal is input to the sixth wiring126. Thus, the second transistor 102 and the sixth transistor 106 areturned off. Because the node 141 at this time is in a floating state,the potential thereof is maintained at V2. Accordingly, the firsttransistor 101 and the fourth transistor 104 remain off. Thus, in thefirst non-selection period, the third wiring 123 is in a floating state,so that the potential of the third wiring 123 is maintained at V2.

In the second non-selection period shown as period E of FIG. 2 and shownin FIG. 3E, the fifth transistor 105 remains off because the signal 221remains at the L level, and the seventh transistor 107 remains offbecause the signal 222 remains at the L level. The potential of the node142 at this time is V1−Vth(103) because the H signal is input to thesixth wiring 126 and the fourth transistor 104 is turned off. Thus, thesecond transistor 102 and the sixth transistor 106 are turned on. Thepotential of the node 141 at this time remains at V2 because thepotential of the tenth wiring 130 (V2) is supplied through the sixthtransistor 106. Accordingly, the first transistor 101 and the fourthtransistor 104 remain off. Thus, in the second non-selection period, thethird wiring 123 is electrically connected to the fourth wiring 124 towhich V2 is supplied, so that the potential of the third wiring 123remains at V2. Thus, the L signal is output through the third wiring123.

Accordingly, the flip-flop of FIG. 1 can make the potential of the thirdwiring 123 V1 by making the potential of the node 141 higher thanV1+Vth(101) using a bootstrap operation in the selection period.Further, the flip-flop of FIG. 1 can obtain merits such as reductions inlayout area and number of elements because the bootstrap operation isperformed using a capacitive coupling of a parasitic capacitance betweenthe second electrode and the gate electrode of the first transistor 101.

Furthermore, the flip-flop of FIG. 1 can suppress shifts in thresholdvoltage of the second transistor 102 and the sixth transistor 106because the second transistor 102 and the sixth transistor 106 areturned on only in the second non-selection period during the first andsecond non-selection periods.

Note that the flip-flop of FIG. 1 can also suppress a shift in thresholdvoltage of the third transistor 103 by supplying V1 to the gateelectrode of the third transistor 103 and inputting the second clocksignal to the first electrode.

In addition, the flip-flop of FIG. 1 can suppress shifts in thresholdvoltage of the first transistor 101, the fourth transistor 104, thefifth transistor 105, and the seventh transistor 107 because the firsttransistor 101, the fourth transistor 104, the fifth transistor 105, andthe seventh transistor 107 are not turned on in the first non-selectionperiod and the second non-selection period.

Further, the flip-flop of FIG. 1 can reset the potentials of the node141 and the third wiring 123 to V2 by supplying V2 to the node 141 andthe third wiring 123 in the second non-selection period even if thepotentials of the node 141 and the third wiring 123 fluctuate in thefirst non-selection period. Thus, the flip-flop of FIG. 1 can suppress amalfunction of which cause is that the node 141 and the third wiring 123are in a floating state and the potentials of the node 141 and the thirdwiring 123 fluctuate.

Furthermore, because the flip-flop of FIG. 1 can suppress a shift inthreshold voltage of a transistor, the flip-flop can suppress amalfunction of which cause is a shift in threshold voltage of atransistor.

Moreover, in the flip-flop of FIG. 1, all of the first to seventhtransistors 101 to 107 are n-channel transistors. Thus, amorphoussilicon can be used for a semiconductor layer of each transistor of theflip-flop of FIG. 1, so that simplification of a manufacturing processcan be achieved, and a reduction in manufacturing cost and animprovement in yield can be achieved. Further, a large-scale displaydevice can also be manufactured. Note that simplification of amanufacturing process can be achieved even if all n-channel transistorsare formed using polysilicon or single crystalline silicon for asemiconductor layer of each transistor.

Note that even if amorphous silicon which exhibits notablecharacteristic deterioration (shift in threshold voltage) is used for asemiconductor layer of each transistor, the flip-flop of FIG. 1 cansuppress characteristic deterioration of each transistor. Therefore, adisplay device with long life can be manufactured.

Here, functions of the first to seventh transistors 101 to 107 aredescribed. The first transistor 101 functions to select timing forsupplying the potential of the fifth wiring 125 to the third wiring 123and to increase the potential of the node 141 by a bootstrap operation,and functions as a bootstrap transistor. The second transistor 102functions to select timing for supplying the potential of the fourthwiring 124 to the third wiring 123 and functions as a switchingtransistor. The third transistor 103 functions to divide a potentialdifference between the potential of the sixth wiring 126 and thepotential of the ninth wiring 129 and functions as a resistor or atransistor having a resistance. The fourth transistor 104 functions toselect timing for supplying the potential of the ninth wiring 129 to thenode 142 and functions as a switching transistor. The fifth transistor105 functions to select timing for supplying the potential of the eighthwiring 128 to the node 141 and functions as an input transistor. Thesixth transistor 106 functions to select timing for supplying thepotential of the tenth wiring 130 to the node 141 and functions as aswitching transistor. The seventh transistor 107 functions to selecttiming for supplying the potential of the eleventh wiring 131 to thenode 141 and functions as a switching transistor. Note that the first toseventh transistors 101 to 107 are not limited to transistors and may beany other elements that have the above-described functions. For example,the second transistor 102, the fourth transistor 104, the sixthtransistor 106, and the seventh transistor 107 each functioning as theswitching transistor may be any element that has a switching function,such as a diode, a CMOS analog switch, or various logic circuits. Thefifth transistor 105 functioning as the input transistor may be anyelement that functions to select timing for being turned off byincreasing the potential of the node 141, such as a PN junction diode ora diode-connected transistor.

Note that the third transistor 103 and the fourth transistor 104constitute an AC pulse generating circuit. The AC pulse generatingcircuit outputs the signal input through the first electrode of thethird transistor 103 to the node 142. Note that the AC pulse generatingcircuit outputs the L signal to the node 142 regardless of the signalinput through the first electrode of the third transistor 103 when thegate electrode of the fourth transistor 104 is at the H level.

In the flip-flop of this embodiment mode, fall time and rise time of thesignal 223 can be shortened when a value of W/L of the first transistor101 is the highest among those of the first to seventh transistors 101to 107. Accordingly, the flip-flop of this embodiment mode can output asignal with less distortion or delay even if a large load is connectedto the third wiring 123.

In the flip-flop of this embodiment mode, a value of W/L of the firsttransistor 101 is preferably twice to five times, more preferably threeto four times as high as that of the fifth transistor 105. Accordingly,the flip-flop of this embodiment mode can output a signal with lessdistortion or delay even if a large load is connected to the thirdwiring 123.

In the flip-flop of this embodiment mode, the potential of the node 142in the set period can be decreased when a value of W/L of the fourthtransistor 104 is greater than that of the third transistor 103.Accordingly, the flip-flop of this embodiment mode can suppress amalfunction because the sixth transistor 106 can surely be turned off inthe set period.

In the flip-flop of this embodiment mode, a value of L of the thirdtransistor 103 is preferably higher than, more preferably twice to threetimes as high as that of the fourth transistor 104. Accordingly, theflip-flop of this embodiment mode can decrease a value of W of thefourth transistor 104 because a value of W/L of the third transistor 103is decreased and can achieve a reduction in layout area.

The arrangement, the number, and the like of the transistors are notlimited to those in FIG. 1 as long as an operation similar to FIG. 1 isachieved. In this embodiment mode, as is apparent from FIGS. 3A to 3Eillustrating the operations of the flip-flop in FIG. 1, electricalconnections in the set period, the selection period, the reset period,the first non-selection period, and the second non-selection period areachieved as indicated by solid lines in FIGS. 3A to 3E, respectively.Accordingly, a transistor, an element (e.g., a resistor or a capacitor),a diode, a switch, various logic circuits, or the like may beadditionally provided if the flip-flop has a structure in whichtransistors or the like are arranged and operated to satisfy the aboveconditions.

For example, a flip-flop shown in FIG. 4A can perform a more stablebootstrap operation in the selection period when a capacitor 401 isprovided between the gate electrode and the second electrode of thefirst transistor 101. In addition, in the flip-flop in FIG. 4A, aparasitic capacitance between the gate electrode and the secondelectrode of the first transistor 101 can be decreased; thus, eachtransistor can be switched at high speed. Alternatively, the capacitor401 may be replaced by a transistor 402 as shown in FIG. 4B. Thetransistor 402 can function as a capacitor with a large capacity when agate electrode is connected to the node 141 and a first electrode and asecond electrode are connected to the third wiring 123. Note that thetransistor 402 can function as a capacitor even when one of the firstand second electrodes is in a floating state. Note that components incommon with those in FIG. 1 are denoted by common reference numerals,and the description is omitted.

Note that the capacitor 401 may use a gate insulating film as aninsulating layer, and a gate electrode layer and a wiring layer asconductive layers; a gate insulating film as an insulating layer, and agate electrode layer and a semiconductor layer to which an impurity isadded as conductive layers; or an interlayer film (insulating film) asan insulating layer, and a wiring layer and a transmissive electrodelayer as conductive layers. Note that when the capacitor 401 uses a gateelectrode layer and a wiring layer as conductive layers, the gateelectrode layer is preferably connected to the gate electrode of thefirst transistor 101 and the wiring layer is preferably connected to thesecond electrode of the first transistor 101. More preferably, when thecapacitor 401 uses a gate electrode layer and a wiring layer asconductive layers, the gate electrode layer is directly connected to thegate electrode of the first transistor 101 and the wiring layer isdirectly connected to the second electrode of the first transistor 101.This is because an increase in layout area of the flip-flop due to thearrangement of the capacitor 401 can be reduced.

Another example is a flip-flop shown in FIG. 4C. By connection of thefirst electrode of the fifth transistor 105 to the first wiring 121 (bydiode-connection of the first transistor 101), the eighth wiring 128becomes unnecessary. Thus, one wiring and power supply (V1) can bereduced. Note that components in common with those in FIG. 1 are denotedby common reference numerals, and the description is omitted.

Another example is a flip-flop shown in FIG. 4D. With the use of aresistor 403 instead of the third transistor 103, one wiring and powersupply can be reduced. In addition, the flip-flop of FIG. 4D can makethe potential of the node 142 equal to the potential of the sixth wiring126 (V1) in the second non-selection period, so that drive capability ofthe flip-flop can be improved. Note that components in common with thosein FIG. 1 are denoted by common reference numerals, and the descriptionis omitted.

Another example is a flip-flop shown in FIG. 7A. By connection of thegate electrode of the second transistor 102 to a wiring 711 to which agiven signal is input, a reverse bias can be applied to the gateelectrode of the second transistor 102. In addition, Vgs of the secondtransistor 102 can be reduced. Thus, a shift in threshold voltage of thesecond transistor 102 can further be suppressed. Note that components incommon with those in FIG. 1 are denoted by common reference numerals,and the description is omitted.

Another example is a flip-flop shown in FIG. 7B. By connection of thegate electrode of the second transistor 102 to the sixth wiring 126, thesecond transistor 102 can be turned on also in the set period.Therefore, drive capability can be improved. In addition, noise of thethird wiring 123 can be reduced. Note that components in common withthose in FIG. 1 are denoted by common reference numerals, and thedescription is omitted.

Another example is a flip-flop shown in FIG. 7C. With the use of adiode-connected transistor 701 and a diode-connected transistor 702instead of the third transistor 103, one wiring and power supply can bereduced. A first electrode of the transistor 701, a second electrode ofthe transistor 702, and a gate electrode of the transistor 701 areconnected to the sixth wiring 126. A second electrode of the transistor701, a first electrode of the transistor 702, and a gate electrode ofthe transistor 702 are connected to the node 141. In other words, tworeversed diodes are connected in parallel between the sixth wiring 126and the node 141. Note that components in common with those in FIG. 1are denoted by common reference numerals, and the description isomitted.

In another example, the sixth transistor 106 is not necessarily needed,as shown in FIG. 21A, when the potential of the node 141 can bemaintained at the L level in the non-selection period. Therefore,because the number of transistors can be reduced, the flip-flop of FIG.21A can obtain a merit such as a reduction in layout area. Note thatcomponents in common with those in FIG. 1 are denoted by commonreference numerals, and the description is omitted.

In another example, the fourth transistor 104 may be replaced by aneighth transistor 2108 as shown in FIG. 21B. A first electrode of theeighth transistor 2108 is connected to a twelfth wiring 2132; a secondelectrode of the eighth transistor 2108 is connected to the node 142;and a gate electrode of the eighth transistor 2108 is connected to thefirst wiring 121. In addition, V2 is supplied to the twelfth wiring2132. Accordingly, whether the eighth transistor 2108 is turned on oroff is controlled by the start signal. Therefore, the flip-flop of FIG.21B can shorten fall time of potential of the node 142 in the set periodand can make the second transistor 102 and the sixth transistor 106turned off in less time. In addition, since the sixth transistor 106 ismade to be turned off in less time, the flip-flop of FIG. 21B canshorten rise time of the potential of the node 141 in the set period.Thus, drive capability of the flip-flop of FIG. 21B can be improved.Note that components in common with those in FIG. 1 are denoted bycommon reference numerals, and the description is omitted.

Note that the eighth wiring 128 may be connected to the fourth wiring124, the ninth wiring 129, the tenth wiring 130, or the eleventh wiring131.

In another example, the eighth transistor 2108 may be additionallyprovided as shown in FIG. 21C. It is acceptable as long as the potentialof the node 142 is at the L level when the start signal is at the Hlevel; therefore, the size of the eighth transistor 2108 can be small.In addition, whether the eighth transistor 2108 is turned on or off iscontrolled by the start signal; therefore, drive capability of theflip-flop of FIG. 21C can be improved similarly to the flip-flop of FIG.21B. Note that components in common with those in FIG. 1 and FIG. 21Bare denoted by common reference numerals, and the description isomitted.

Note that the connection relationship of the wirings is not limited tothat in FIG. 1 as long as a similar operation to FIG. 1 is achieved. Asis apparent from FIGS. 3A to 3E illustrating the operations of theflip-flop in FIG. 1, in this embodiment mode, electrical connections inthe set period, the selection period, the reset period, the firstnon-selection period, and the second non-selection period are achievedas indicated by solid lines in FIGS. 3A to 3E, respectively. Thus, thewirings may be provided or connected to satisfy the above conditions.

For example, the first electrode of the second transistor 102, the firstelectrode of the fourth transistor 104, the first electrode of the sixthtransistor 106, and the first electrode of the seventh transistor 107may be connected to a sixth wiring 506 as shown in FIG. 5A. In addition,the gate electrode of the third transistor 103 and the first electrodeof the fifth transistor 105 may be connected to a seventh wiring 507.Thus, in the flip-flop of FIG. 5A, the number of wirings can be reducedfrom eleven to seven as compared with the flip-flop of FIG. 1. Further,because the number of wirings of the flip-flop in FIG. 5A can bereduced, a yield of a shift register can be improved. Furthermore, inthe flip-flop of FIG. 5A, an area for leading wirings can be decreasedand a layout area of a shift register can be reduced. Moreover, in theflip-flop of FIG. 5A, the width of each wiring can be increased, so thata voltage drop can be reduced and drive capability of a shift registercan be improved. Note that components in common with those in FIG. 1 aredenoted by common reference numerals, and the description is omitted.

Note that the sixth wiring 506 shown in FIG. 5A corresponds to thefourth wiring 124, the ninth wiring 129, the tenth wiring 130, and theeleventh wiring 131 shown in FIG. 1. The seventh wiring 507 shown inFIG. 5A corresponds to the seventh wiring 127 and the eighth wiring 128shown in FIG. 1. A first wiring 505, a second wiring 502, a third wiring503, a fourth wiring 504, and a fifth wiring 505 shown in FIG. 5Acorrespond to the first wiring 121, the second wiring 122, the thirdwiring 123, the fifth wiring 125, and the sixth wiring 126 shown in FIG.1, respectively.

The sixth wiring 506 and the seventh wiring 507 may be referred to as afirst power supply line and a second power supply line, respectively.The first wiring 501, the second wiring 502, the third wiring 503, thefourth wiring 504, and the fifth wiring 505 may be referred to as afirst signal line, a second signal line, a third signal line, a fourthsignal line, and a fifth signal line, respectively.

In another example, the first electrode of the fourth transistor 104 maybe connected to an eighth wiring 508 as shown in FIG. 5B. The flip-flopof FIG. 5B can suppress a malfunction due to voltage drop of the sixthwiring 506 by allowing an instantaneous current generated in the fourthtransistor 104 in the set period to flow to the eighth wiring 508. Notethat components in common with those in FIG. 1 and FIG. 5A are denotedby common reference numerals, and the description is omitted.

In another example, the first electrode of the second transistor 102 maybe connected to a ninth wiring 509 as shown in FIG. 5C. The flip-flop ofFIG. 5C can suppress a malfunction due to voltage drop of the sixthwiring 506 by allowing an instantaneous current generated in the secondtransistor 102 in the reset period to flow to the ninth wiring 509. Notethat components in common with those in FIG. 1 and FIG. 5A are denotedby common reference numerals, and the description is omitted.

In another example, the gate electrode of the third transistor 103 maybe connected to a tenth wiring 510 as shown in FIG. 5D. The flip-flop ofFIG. 5D can suppress characteristic deterioration of the secondtransistor 102 and the sixth transistor 106 because the potential of thegate electrode of the second transistor 102 and the potential of thegate electrode of the sixth transistor 106 can be lowered in the secondnon-selection period by supplying a potential lower than V1 to the tenthwiring 510. Note that components in common with those in FIG. 1 and FIG.5A are denoted by common reference numerals, and the description isomitted.

Note that the power supply potential, signal amplitude, and signaltiming are not limited by the timing chart of FIG. 2 as long as asimilar operation to FIG. 1 is achieved. As is apparent from FIGS. 3A to3E illustrating the operations of the flip-flop in FIG. 1, in thisembodiment mode, electrical connections in the set period, the selectionperiod, the reset period, the first non-selection period, and the secondnon-selection period are achieved as indicated by solid lines in FIGS.3A to 3E, respectively. Thus, the power supply potential, signalamplitude, and signal timing may be changed to satisfy the aboveconditions.

For example, periods for inputting the H signal to the first wiring 121,the fifth wiring 125, and the sixth wiring 126 may be shorter as shownin a timing chart of FIG. 6. In FIG. 6, as compared with the timingchart of FIG. 2, timing at which a signal is switched from L level to Hlevel is delayed for a period Ta1, and timing at which a signal isswitched from H level to L level is advanced for a period Ta2. In otherwords, in FIG. 6, as compared with FIG. 2, a period in which a signal isat the H level (period Th) is shorter by (the period Ta1+the periodTa2). Thus, in a flip-flop to which the timing chart of FIG. 6 isapplied, instantaneous current through each wiring is reduced, so thatpower saving, suppression of malfunction, improvement of drivecapability, and the like can be realized. Further, in the flip-flop towhich the timing chart of FIG. 6 is applied, fall time of the signaloutput through the third wiring 123 can be shortened in the resetperiod. This is because timing at which the potential of the node 141becomes L level is delayed for (the period Ta1+the period Ta2), so thatan L signal input to the fifth wiring 125 is supplied to the thirdwiring 123 through the first transistor 101 with high current supplycapability (with a large channel width). Note that portions in commonwith those in the timing chart of FIG. 2 are denoted by common referencenumerals, and the description is omitted.

Note that the relationship between the period Ta1, the period Ta2, andthe period Tb preferably satisfies ((Ta1+Tb)/(Ta1+Ta2+Tb))×100<10[%].More preferably, the relationship satisfies((Ta1+Tb)/(Ta1+Ta2+Tb))×100<5[%]. Still more preferably, therelationship of (the period Ta1≈the period Ta2) is satisfied.

In another example, shifts in threshold voltage of the second transistor102 and the sixth transistor 106 can be suppressed because the potentialof the node 142 is Va−Vth(103) in the reset period and the secondnon-selection period if Va (V2<Va<V1) is supplied to the seventh wiring127.

In another example, the second transistor 102 and the sixth transistor106 can easily be turned on because the potential of the node 142 is V1in the reset period and the second non-selection period if Vb(V1+Vth(103)<Vb) is supplied to the seventh wiring 127.

In another example, shifts in threshold voltage of the second transistor102 and the sixth transistor 106 can be suppressed by inputting the Lsignal with a potential of Vc (Vc<V2) and the H signal with a potentialof Vd (V1>Vd>V2) to the sixth wiring 126. This is because the potentialof the node 142 is Vc in the set period and the first non-selectionperiod, so that a reverse bias is applied to the second transistor 102and the sixth transistor 106. Another reason is that the potential ofthe node 142 is Vd in the reset period and the second non-selectionperiod, so that Vgs of the second transistor 102 and the sixthtransistor 106 is decreased.

FIG. 25 shows an example of a top view of the flip-flop shown in FIG.5A. A conductive layer 2501 includes portions functioning as the gateelectrode of the second transistor 102 and the gate electrode of thesixth transistor 106 and is connected to a conductive layer 2502 througha wiring 2550. The conductive layer 2502 includes portions functioningas the second electrode of the third transistor 103 and the secondelectrode of the fourth transistor 104. A conductive layer 2503 includesportions functioning as the first electrode of the second transistor102, the first electrode of the sixth transistor 106, and the firstelectrode of the fourth transistor 104 and is connected to the sixthwiring 506. A conductive layer 2504 includes a portion functioning asthe second electrode of the second transistor 102 and is connected tothe third wiring 503 through a wiring 2548. A conductive layer 2505includes portions functioning as the second electrode of the fifthtransistor 105 and the second electrode of the seventh transistor 107and is connected to a conductive layer 2510 through a wiring 2549. Aconductive layer 2506 includes a portion functioning as the firstelectrode of the seventh transistor 107 and is connected to the sixthwiring 506. A conductive layer 2507 includes a portion functioning asthe first electrode of the first transistor 101 and is connected to thefourth wiring 504 through a wiring 2541. A conductive layer 2508includes a portion functioning as the second electrode of the firsttransistor 101 and is connected to the third wiring 503 through thewiring 2548. A conductive layer 2509 includes a portion functioning asthe first electrode of the fifth transistor 105 and is connected to theseventh wiring 507 through a wiring 2542. The conductive layer 2510includes portions functioning as the gate electrode of the firsttransistor 101 and the gate electrode of the fourth transistor 104. Aconductive layer 2511 includes a portion functioning as the gateelectrode of the seventh transistor 107 and is connected to the secondwiring 502 through a wiring 2546. A conductive layer 2512 includes aportion functioning as the gate electrode of the third transistor 103and is connected to the seventh wiring 507 through a wiring 2544. Aconductive layer 2513 includes a portion functioning as the firstelectrode of the third transistor 103 and is connected to the fifthwiring 505 through a wiring 2543. A conductive layer 2514 includes aportion functioning as the gate electrode of the fifth transistor 105and is connected to the first wiring 501 through a wiring 2545. Aconductive layer 2515 includes a portion functioning as the secondelectrode of the sixth transistor 106 and is connected to the conductivelayer 2510 through a wiring 2547.

Here, the width of the wiring 2546 is narrower than that of the wiring2541, 2542, 2543, 2544, 2545, 2547, 2548, 2549, or 2550. Alternatively,the length of the wiring 2546 is long. In other words, the wiring 2546has a high resistance. Accordingly, timing at which a potential of theconductive layer 2511 becomes H level can be delayed in the resetperiod. Thus, timing at which the seventh transistor 107 is turned oncan be delayed in the reset period, so that a signal of the third wiring503 can become L level in a shorter period. This is because timing atwhich the node 141 becomes L level is delayed, and in this delay period,the L signal is supplied to the third wiring 503 through the firsttransistor 101.

Note that the wirings 2541, 2542, 2543, 2544, 2545, 2546, 2547, 2548,2549, and 2550 are similar to pixel electrodes (also referred to aslight-transmitting electrodes or reflective electrodes) and are formedusing a similar process and material thereto.

Note that the portions functioning as the gate electrode, the firstelectrode, and the second electrode of the first transistor 101 areportions where the conductive layers including each electrode overlapwith a semiconductor layer 2581. The portions functioning as the gateelectrode, the first electrode, and the second electrode of the secondtransistor 102 are portions where the conductive layers including eachelectrode overlap with a semiconductor layer 2582. The portionsfunctioning as the gate electrode, the first electrode, and the secondelectrode of the third transistor 103 are portions where the conductivelayers including each electrode overlap with a semiconductor layer 2583.The portions functioning as the gate electrode, the first electrode, andthe second electrode of the fourth transistor 104 are portions where theconductive layers including each electrode overlap with a semiconductorlayer 2584. The portions functioning as the gate electrode, the firstelectrode, and the second electrode of the fifth transistor 105 areportions where the conductive layers including each electrode overlapwith a semiconductor layer 2585. The portions functioning as the gateelectrode, the first electrode, and the second electrode of the sixthtransistor 106 are portions where the conductive layers including eachelectrode overlap with a semiconductor layer 2586. The portionsfunctioning as the gate electrode, the first electrode, and the secondelectrode of the seventh transistor 107 are portions where theconductive layers including each electrode overlap with a semiconductorlayer 2587.

A structure and a driving method of a shift register including theaforementioned flip-flop of this embodiment mode are described.

A structure of a shift register of this embodiment mode is describedwith reference to FIG. 10. The shift register in FIG. 10 includes nflip-flops (flip-flops 1001_1 to 1001 _(—) n).

The connection relationship of the shift register in FIG. 10 isdescribed. In the shift register in FIG. 10, a flip-flop 1001 _(—) i inan i-th stage (one of the flip-flops 1001_1 to 1001 _(—) n) is connectedto a second wiring 1012, a third wiring 1013, a fourth wiring 1014, afifth wiring 1015, a sixth wiring 1016, an eighth wiring 1018 _(—) i−1,an eighth wiring 1018 _(—) i, and an eighth wiring 1018 _(—) i+1. Notethat the flip-flop 1001_1 in the first stage is connected to a firstwiring 1011, the second wiring 1012, the third wiring 1013, the fourthwiring 1014, the fifth wiring 1015, the sixth wiring 1016, an eighthwiring 1018_1, and an eighth wiring 1018_2. The flip-flop 1001 _(—) n inan n-th stage is connected to the second wiring 1012, the third wiring1013, the fourth wiring 1014, the fifth wiring 1015, the sixth wiring1016, the seventh wiring 1017, an eighth wiring 1018 _(—) n−1, and aneighth wiring 1018 _(—) n.

The first wiring 1011 is connected to the first wiring 121 shown in FIG.1 of the flip-flop 1001_1. The second wiring 1012 is connected to thefifth wiring 125 shown in FIG. 1 of a flip-flop in an odd-numberedstage, and is connected to the sixth wiring 126 shown in FIG. 1 of aflip-flop in an even-numbered stage. The third wiring 1013 is connectedto the sixth wiring 126 shown in FIG. 1 of a flip-flop in anodd-numbered stage, and is connected to the fifth wiring 125 shown inFIG. 1 of a flip-flop in an even-numbered stage. The fourth wiring 1014is connected to the seventh wiring 127 shown in FIG. 1 of a flip-flop inevery stage. The fifth wiring 1015 is connected to the eighth wiring 128shown in FIG. 1 of a flip-flop of every stage. The sixth wiring 1016 isconnected to the fourth wiring 124, the ninth wiring 129, the tenthwiring 130, and the eleventh wiring 131 shown in FIG. 1 of a flip-flopof every stage. The eighth wiring 1018 _(—) i is connected to the secondwiring 122 shown in FIG. 1 of the flip-flop_i−1, the third wiring 123shown in FIG. 1 of the flip-flop 1001 _(—) i, and the first wiring 121shown in FIG. 1 of the flip-flop 1001 _(—) i+1. Note that the eighthwiring 1018_1 is connected to the third wiring 123 shown in FIG. 1 ofthe flip-flop 1001_1 and the first wiring 121 shown in FIG. 1 of theflip-flop 1001_2. The eighth wiring 1018 _(—) n is connected to thesecond wiring 122 shown in FIG. 1 of the flip-flop 1001 _(—) n−1 and thethird wiring 123 shown in FIG. 1 of the flip-flop 1001 _(—) n.

The fourth wiring 1014 and the fifth wiring 1015 are each supplied withthe potential V1, and the sixth wiring 1016 is supplied with thepotential V2.

Signals are input to the first wiring 1011, the second wiring 1012, thethird wiring 1013, and the seventh wiring 1017. The signal input to thefirst wiring 1011 is a start signal; the signal input to the secondwiring 1012 is a first clock signal; the signal input to the thirdwiring 1013 is a second clock signal; and the signal input to theseventh wiring 1017 is a reset signal. In addition, each of the signalsinput to the first wiring 1011, the second wiring 1012, the third wiring1013, and the seventh wiring 1017 is a digital signal including an Hsignal with a potential of V1 and an L signal with a potential of V2.

Various signals, power supply potentials, or currents may be input tothe first to seventh wirings 1011 to 1017.

Signals are output through the eighth wirings 1018_1 to 1018 _(—) n. Forexample, the signal output through the eighth wiring 1018 _(—) i is anoutput signal of the flip-flop 1001 _(—) i. Further, the signal outputthrough the eighth wiring 1018 _(—) i is a start signal of the flip-flop1001 _(—) i+1 and a reset signal of the flip-flop 1001 _(—) i−1.

Note that when the same signal is input to or the same voltage issupplied to the first to seventh wirings 1011 to 1017, the first toseventh wirings 1011 to 1017 may be connected to each other or may be asingle wiring.

Next, the operation of the shift register shown in FIG. 10 is describedwith reference to a timing chart of FIG. 11 and a timing chart of FIG.12. The timing chart of FIG. 11 is divided into a scan period and aretrace period. The scan period corresponds to a period from the timewhen output of a selection signal from the eighth wiring 1018_1 startsto the time when output of a selection signal from the eighth wiring1018 _(—) n ends. The retrace period corresponds to a period from thetime when output of the selection signal from the eighth wiring 1018_(—) n ends to the time when output of the selection signal from theeighth wiring 1018_1 starts.

Note that FIG. 11 shows a signal 1111 input to the first wiring 1011, asignal 1112 input to the second wiring 1012, a signal 1113 input to thethird wiring 1013, a signal 1117 input to the seventh wiring 1017, asignal 1118_1 output to the eighth wiring 1018_1, a signal 1118_2 outputto the eighth wiring 1018_2, and a signal 1118 _(—) n output to theeighth wiring 1018 _(—) n. FIG. 12 shows a signal 1211 input to thefirst wiring 1011, a signal 1218_1 output through the eighth wiring1018_1, a signal 1218 _(—) i output to the eighth wiring 1018 _(—) i, asignal 1218 _(—) i+1 output to the eighth wiring 1018 _(—) i+1, and asignal 1218 _(—) n output to the eighth wiring 1018 _(—) n.

As shown in FIG. 12, if the flip-flop 1001 _(—) i, for example, is inthe selection period, the H signal is output to the eighth wiring 1018_(—) i. At this time, the flip-flop 1001 _(—) i+1 is in the set period.Subsequently, the flip-flop 1001 _(—) i is in the reset period, and theL signal is output through the eighth wiring 1018 _(—) i. At this time,the flip-flop 1001 _(—) i+1 is in the selection period. After that, theflip-flop 1001 _(—) i is in the first non-selection period, and theeighth wiring 1018 _(—) i is in a floating state to maintain thepotential at the L level. At this time, the flip-flop 1001 _(—) i+1 isin the reset period. After that, the flip-flop 1001 _(—) i is in thesecond non-selection period, and the L signal is output through theeighth wiring 1018 _(—) i. At this time, the flip-flop 1001 _(—) i+1 isin the first non-selection period. Thus, the flip-flop 1001 _(—) irepeats the first non-selection period and the second non-selectionperiod until the next set period.

Accordingly, the shift register of FIG. 10 can output a selection signalsequentially through the eighth wirings 1018_1 to 1018 _(—) n. In otherwords, the shift register of FIG. 10 can scan the eighth wirings 1018_1to 1018 _(—) n. Therefore, the shift register of FIG. 10 can perform asufficient function as a shift register.

In addition, the reset signal input to the flip-flop 1001 _(—) n in thelast stage is characterized by being input through the seventh wiring1017. Accordingly, a dummy flip-flop becomes unnecessary for the shiftregister of FIG. 10, so that a layout area can be reduced. However, adummy flip-flop may be provided.

For the shift register of FIG. 10, the retrace period can be freelydetermined depending on timing of the signal input to the first wiring1011.

The shift register of FIG. 10 can suppress a shift in threshold voltageof a transistor by employing the flip-flop described in this embodimentmode. In addition, the shift register of FIG. 10 can have a longer life,can improve drive capability, can suppress a malfunction, and cansimplify a process.

Note that the shift register is not limited to the structure of FIG. 10if a similar operation to FIG. 10 is achieved.

For example, output signals of the flip-flops may each be output throughbuffers as shown in FIG. 13. Because the flip-flops 1001_1 to 1001 _(—)n are connected to the eighth wirings 1018_1 to 1018 _(—) n throughbuffers 1301_1 to 1301 _(—) n, respectively, a shift register of FIG. 13can have high drive capability. This is because if a large load isconnected to each of the eighth wirings 1018_1 to 1018 _(—) n, a signaloutput through each of the eighth wirings 1018_1 to 1018 _(—) n isdelayed or distorted. In other words, this is because the delay ordistortion of the signal output through each of the eighth wirings1018_1 to 1018 _(—) n does not affect the operation of the shiftregister. Note that components in common with those in FIG. 10 aredenoted by common reference numerals, and the description is omitted.

Note that each of the buffers 1301_1 to 1301 _(—) n may be a logiccircuit such as NAND or NOR, an operational amplifier, or a combinationof these. In other words, it may be an inverter, an analog buffer, orthe like. Further, each of the buffers 1301_1 to 1301 _(—) n ispreferably formed of an n-channel transistor if each flip-flop is formedof an n-channel transistor. Furthermore, each of the buffers 1301_1 to1301 _(—) n preferably has such a structure that enables a bootstrapoperation. Moreover, a drive voltage (a potential difference between apositive power supply and a negative power supply) of each of thebuffers 1301_1 to 1301 _(—) n is preferably higher than that of each ofthe flip-flops 1001_1 to 1001 _(—) n.

Examples of the buffers 1301_1 to 1301 _(—) n included in the shiftregister of FIG. 13 are described with reference to FIGS. 105A and 105B.In a buffer 8000 shown in FIG. 105A, inverters 8001 a, 8001 b, and 8001c are connected between wirings 8011 and 8012. Accordingly, an invertedsignal of a signal input to the wiring 8011 is output through the wiring8012. Note that the number of inverters connected between the wirings8011 and 8012 is not limited. For example, if an even number ofinverters are connected between the wirings 8011 and 8012, a signal withthe same polarity as that of a signal input to the wiring 8011 is outputthrough the wiring 8012. In addition, as shown in a buffer 8100 of FIG.105B, inverters 8002 a, 8002 b, and 8002 c connected in series andinverters 8003 a, 8003 b, and 8003 c connected in series may beconnected in parallel. In the buffer 8100 of FIG. 105B, since variationof in characteristics of transistors can be averaged, delay anddistortion of the signal output through the wiring 8012 can be reduced.Further, outputs of the inverters 8002 a and 8003 a, and outputs of theinverters 8002 b and 8003 b may be connected to each other.

In FIG. 105A, it is preferable to satisfy (W of a transistor included inthe inverter 8001 a)<(W of a transistor included in the inverter 8001b)<(W of a transistor included in the inverter 8001 c). This is becausedrive capability of a flip-flop (specifically, a value of W/L of thetransistor 101 in FIG. 1) can be small if W of the transistor includedin the inverter 8001 a is small; thus, a layout area of the shiftregister of this embodiment mode can be decreased. Similarly, in FIG.105B, it is preferable to satisfy (W of a transistor included in theinverter 8002 a)<(W of a transistor included in the inverter 8002 b)<(Wof a transistor included in the inverter 8002 c). Similarly, in FIG.105B, it is preferable to satisfy (W of a transistor included in theinverter 8003 a)<(W of a transistor included in the inverter 8003 b)<(Wof a transistor included in the inverter 8003 c). Further, it ispreferable to satisfy (W of the transistor included in the inverter 8002a)=(W of the transistor included in the inverter 8003 a), (W of thetransistor included in the inverter 8002 b)=(W of the transistorincluded in the inverter 8003 b), and (W of the transistor included inthe inverter 8002 c)=(W of the transistor included in the inverter 8003c).

The inverters shown in FIGS. 105A and 105B are not particularly limitedas long as they can output an inverted signal of an input signal. Forexample, as shown in FIG. 105C, an inverter may be formed of a firsttransistor 8201 and a second transistor 8202. A signal is input to afirst wiring 8211, and a signal is output through a second wiring 8212.V1 is supplied to a third wiring 8213, and V2 is supplied to a fourthwiring 8214. When the H signal is input to the first wiring 8211, theinverter of FIG. 105C outputs a potential obtained by dividing V1−V2 bythe first transistor 8201 and the second transistor 8202 ((W/L of thefirst transistor 8201)<(W/L of the second transistor 8202)) through thesecond wiring 8212. Further, when the L signal is input to the firstwiring 8211, the inverter of FIG. 105C outputs V1−Vth(8201) (Vth(8201):a threshold voltage of the first transistor 8201) through the secondwiring 8212. The first transistor 8201 may be any element having aresistance, such as a PN junction diode or simply a resistor.

As shown in FIG. 105D, an inverter may be formed of a first transistor8301, a second transistor 8302, a third transistor 8303, and a fourthtransistor 8304. A signal is input to a first wiring 8311; a signal isoutput through a second wiring 8312; V1 is supplied to a third wiring8313 and a fifth wiring 8315; and V2 is supplied to a fourth wiring 8314and a sixth wiring 8316. When the H signal is input to the first wiring8311, the inverter of FIG. 105D outputs V2 through the second wiring8312. At this time, a potential of a node 8341 is at the L level, sothat the first transistor 8301 is turned off. Further, when the L signalis input to the first wiring 8311, the inverter of FIG. 105D outputs V1through the second wiring 8312. At this time, when the potential of thenode 8341 becomes V1−Vth(8303) (Vth(8303): a threshold voltage of thethird transistor 8303), the node 8341 is in a floating state, and thepotential of the node 8341 becomes higher than V1+Vth(8301) (Vth(8301):a threshold voltage of the first transistor 8301) by a bootstrapoperation. Thus, the first transistor 8301 is turned on. Further, acapacitor may be provided between a second electrode and a gateelectrode of the first transistor 8301 because the first transistor 8301functions as a bootstrap transistor.

As shown in FIG. 26A, an inverter may be formed of a first transistor8401, a second transistor 8402, a third transistor 8403, and a fourthtransistor 8404. The inverter of FIG. 26A is a two-input inverter andcan perform a bootstrap operation. A signal is input to a first wiring8411; an inverted signal is input to a second wiring 8412; a signal isoutput through a third wiring 8413; V1 is supplied to a fourth wiring8414 and a sixth wiring 8416; and V2 is supplied to a fifth wiring 8415and a seventh wiring 8417. When the L signal is input to the firstwiring 8411 and the H signal is input to the second wiring 8412, theinverter of FIG. 26A outputs V2 through the third wiring 8413. At thistime, a potential of a node 8441 is V2, so that the first transistor8401 is turned off. Further, when the H signal is input to the firstwiring 8411 and the L signal is input to the second wiring 8412, theinverter of FIG. 26A outputs V1 through the third wiring 8413. At thistime, when the potential of the node 8441 becomes V1−Vth(8403)(Vth(8403): a threshold voltage of the third transistor 8403), the node8441 is in a floating state, and the potential of the node 8441 becomeshigher than V1+Vth(8401) (Vth(8401): a threshold voltage of the firsttransistor 8401) by a bootstrap operation. Thus, the first transistor8401 is turned on. Further, a capacitor may be provided between a secondelectrode and a gate electrode of the first transistor 8401 because thefirst transistor 8401 functions as a bootstrap transistor. Furthermore,one of the first wiring 8411 and the second wiring 8412 is preferablyconnected to the third wiring 123 shown in FIG. 1, and the other ispreferably connected to the node 142 shown in FIG. 1.

As shown in FIG. 26B, an inverter may be formed of a first transistor8501, a second transistor 8502, and a third transistor 8503. Theinverter of FIG. 26B is a two-input inverter and can perform a bootstrapoperation. A signal is input to a first wiring 8511; an inverted signalis input to a second wiring 8512; a signal is output through a thirdwiring 8513; V1 is supplied to a fourth wiring 8514 and a sixth wiring8516; and V2 is supplied to a fifth wiring 8515. When the L signal isinput to the first wiring 8511 and the H signal is input to the secondwiring 8512, the inverter of FIG. 26B outputs V2 through the thirdwiring 8513. At this time, a potential of a node 8541 is V2, so that thefirst transistor 8501 is turned off. Further, when the H signal is inputto the first wiring 8511 and the L signal is input to the second wiring8512, the inverter of FIG. 26B outputs V1 through the third wiring 8513.At this time, when the potential of the node 8541 becomes V1−Vth(8503)(Vth(8503): a threshold voltage of the third transistor 8503), the node8541 is in a floating state, and the potential of the node 8541 becomeshigher than V1+Vth(8501) (Vth(8501): a threshold voltage of the firsttransistor 8501) by a bootstrap operation. Thus, the first transistor8501 is turned on. Further, a capacitor may be provided between a secondelectrode and a gate electrode of the first transistor 8501 because thefirst transistor 8501 functions as a bootstrap transistor. Furthermore,one of the first wiring 8511 and the second wiring 8512 is preferablyconnected to the third wiring 123 shown in FIG. 1, and the other ispreferably connected to the node 142 shown in FIG. 1.

As shown in FIG. 26C, an inverter may be formed of a first transistor8601, a second transistor 8602, a third transistor 8603, and a fourthtransistor 8604. The inverter of FIG. 26C is a two-input inverter andcan perform a bootstrap operation. A signal is input to a first wiring8611; an inverted signal is input to a second wiring 8612; a signal isoutput through a third wiring 8613; V1 is supplied to a fourth wiring8614; and V2 is supplied to a fifth wiring 8615 and a sixth wiring 8616.When the L signal is input to the first wiring 8611 and the H signal isinput to the second wiring 8612, the inverter of FIG. 26C outputs V2through the third wiring 8613. At this time, a potential of a node 8641is V2, so that the first transistor 8601 is turned off. Further, whenthe H signal is input to the first wiring 8611 and the L signal is inputto the second wiring 8612, the inverter of FIG. 26C outputs V1 throughthe third wiring 8613. At this time, when the potential of the node 8641becomes V1−Vth(8603) (Vth(8603): a threshold voltage of the thirdtransistor 8603), the node 8641 is in a floating state, and thepotential of the node 8641 becomes higher than V1+Vth(8601) (Vth(8601):a threshold voltage of the first transistor 8601) by a bootstrapoperation. Thus, the first transistor 8601 is turned on. Further, acapacitor may be provided between a second electrode and a gateelectrode of the first transistor 8601 because the first transistor 8601functions as a bootstrap transistor. Furthermore, one of the firstwiring 8611 and the second wiring 8612 is preferably connected to thethird wiring 123 shown in FIG. 1, and the other is preferably connectedto the node 142 shown in FIG. 1.

In another example, the reset signal input to the flip-flop 1001 _(—) nmay be another input signal or output signal of the shift register. Inother words, one wiring and one signal can be reduced by generation ofthe reset signal input to the flip-flop 1001 _(—) n in the shiftregister. For example, when the flip-flop 1001 _(—) n is in aneven-numbered stage, it may be connected to the eighth wiring 1018_1 asshown in FIG. 14. In another example, when the flip-flop 1001 _(—) n isin an even-numbered stage, it may be connected to the first wiring 1011as shown in FIG. 15. In another example, the reset signal input to theflip-flop 1001 _(—) n may be generated using a dummy flip-flop 1001 _(—)d as shown in FIG. 17.

The dummy flip-flop 1001 _(—) d may be a flip-flop similar to theflip-flop 1001 _(—) n−1. Note that the second wiring 122 shown in FIG. 1of the dummy flip-flop 1001 _(—) d is connected to the sixth wiring 1016in FIG. 17. Note that components in common with those in FIG. 10 aredenoted by common reference numerals, and the description is omitted.

Next, a structure and a driving method of a display device including theaforementioned shift register of this embodiment mode are described.Note that a display device of this embodiment mode includes at least theflip-flop of this embodiment mode.

A structure of the display device of this embodiment mode is describedwith reference to FIG. 18. The display device in FIG. 18 includes asignal line driver circuit 1801, a scan line driver circuit 1802, and apixel portion 1804. The pixel portion 1804 includes a plurality ofsignal lines S1 to Sm provided to extend from the signal line drivercircuit 1801 in a column direction, a plurality of scan lines G1 to Gnprovided to extend from the scan line driver circuit 1802 in a rowdirection, and a plurality of pixels 1803 arranged in matrixcorresponding to the signal lines S1 to Sm and the scan lines G1 to Gn.Each pixel 1803 is connected to the signal line Sj (one of the signallines S1 to Sm) and the scan line Gi (one of the scan lines G1 to Gn).

The shift register of this embodiment mode can be applied to the scanline driver circuit 1802. It is needless to say that the shift registerof this embodiment mode can be also used as the signal line drivercircuit 1801.

The scan lines G1 to Gn are connected to the eighth wirings 1018_1 to1018 _(—) n shown in FIGS. 10, 13 to 15, and 17.

The signal lines and the scan lines may be simply referred to aswirings. The signal line driver circuit 1801 and the scan line drivercircuit 1802 may each be referred to as a driver circuit.

The pixel 1803 includes at least a switching element, a capacitor, and apixel electrode. Note that the pixel 1803 may include a plurality ofswitching elements or a plurality of capacitors. Further, a capacitor isnot always needed. The pixel 1803 may include a transistor whichoperates in a saturation region. The pixel 1803 may include a displayelement such as a liquid crystal element or an EL element. As theswitching element, a transistor or a PN junction diode can be used. Whena transistor is used as the switching element, it preferably operates ina linear region. Further, when the scan line driver circuit 1802includes only n-channel transistors, an n-channel transistor ispreferably used as the switching element. When the scan line drivercircuit 1802 includes only p-channel transistors, a p-channel transistoris preferably used as the switching element.

The scan line driver circuit 1802 and the pixel portion 1804 are formedover an insulating substrate 1805, and the signal line driver circuit1801 is not formed over the insulating substrate 1805. The signal linedriver circuit 1801 is formed on a single crystalline substrate, an SOIsubstrate, or over another insulating substrate which is different fromthe insulating substrate 1805. The signal line driver circuit 1801 isconnected to the signal lines S1 to Sm through a printed wiring boardsuch as an FPC. Note that the signal line driver circuit 1801 may beformed over the insulating substrate 1805, or a circuit forming part ofthe signal line driver circuit 1801 may be formed over the insulatingsubstrate 1805.

The signal line driver circuit 1801 inputs a voltage or a current as avideo signal to the signal lines S1 to Sm. Note that the video signalmay be an analog signal or a digital signal. Positive and negativepolarities of the video signal may be inverted for each frame (i.e.,frame inversion driving), may be inverted for each row (i.e., gate lineinversion driving), may be inverted for each column (i.e., source lineinversion driving), or may be inverted for each row and column (i.e.,dot inversion driving). Further, the video signal may be input to thesignal lines S1 to Sm with dot sequential driving or line sequentialdriving. The signal line driver circuit 1801 may input not only thevideo signal but also a certain voltage such as precharge voltage to thesignal lines S1 to Sm. A certain voltage such as precharge voltage ispreferably input in each frame or in each gate selection period.

The scan line driver circuit 1802 inputs a signal to the scan lines G1to Gn and selects (hereinafter also referred to as scans) the scan linesG1 to Gn sequentially from the first row. Then, the scan line drivercircuit 1802 selects the plurality of pixels 1803 connected to theselected scan line. Here, one gate selection period refers to a periodin which one scan line is selected, and a non-selection period refers toa period in which the scan line is not selected. A scan signal refers toa signal output to the scan line from the scan line driver circuit 1802.The maximum value of the scan signal is greater than the maximum valueof the video signal or the maximum voltage of the signal line, and theminimum value of the scan signal is less than the minimum value of thevideo signal or the minimum voltage of the signal line.

When the pixel 1803 is selected, the video signal is input to the pixel1803 from the signal line driver circuit 1801 through the signal line.When the pixel 1803 is not selected, the pixel 1803 maintains the videosignal (a potential corresponding to the video signal) input in theselection period.

Although not shown, a plurality of potentials and a plurality of signalsare supplied to the signal line driver circuit 1801 and the scan linedriver circuit 1802.

Next, an operation of the display device shown in FIG. 18 is describedwith reference to a timing chart of FIG. 19. FIG. 19 shows one frameperiod corresponding to a period for displaying an image for one screen.Although one frame period is not particularly limited, it is preferably1/60 seconds or less so that a person viewing an image does not perceivea flicker.

The timing chart of FIG. 19 shows each timing for selecting the scanline G1 in the first row, the scan line Gi in the i-th row, the scanline Gi+1 in the (i+1)th row, and the scan line Gn in the n-th row.

In FIG. 19, the scan line Gi in the i-th row is selected, for example,and the plurality of pixels 1803 connected to the scan line Gi areselected. Then, a video signal is input to each of the plurality ofpixels 1803 connected to the scan line Gi, and each of the plurality ofpixels 1803 maintains a potential corresponding to the video signal.After that, the scan line Gi in the i-th row is non-selected, the scanline Gi+1 in the (i+1)th row is selected, and the plurality of pixels1803 connected to the scan line Gi+1 are selected. Then, a video signalis input to each of the plurality of pixels 1803 connected to the scanline Gi+1, and each of the plurality of pixels 1803 maintains apotential corresponding to the video signal. Thus, in one frame period,the scan lines G1 to Gn are sequentially selected, and the pixels 1803connected to each scan line are also sequentially selected. A videosignal is input to each of the plurality of pixels 1803 connected toeach scan line, and each of the plurality of pixels 1803 maintains apotential corresponding to the video signal.

Accordingly, the display device of FIG. 18 can input video signalsindependently to all pixels, so that it can sufficiently operate as anactive matrix display device.

Further, in the display device of FIG. 18, the shift register of thisembodiment mode is used as the scan line driver circuit 1802, so that ashift in threshold voltage of a transistor can be suppressed. Thedisplay device of FIG. 18 can obtain a longer life, can improve drivecapability, can suppress malfunction, and can simplify a process.

In the display device of FIG. 18, the signal line driver circuit 1801which needs to operate at high speed is formed over a substratedifferent from that for the scan line driver circuit 1802 and the pixelportion 1804. Therefore, amorphous silicon can be used for semiconductorlayers of transistors included in the scan line driver circuit 1802 andthe pixels 1803. The display device of FIG. 18 achieves simplificationof a manufacturing process, reduction in manufacturing cost, andimprovement in yield. Further, the size of the display device of thisembodiment mode can be increased. Even when polysilicon or singlecrystalline silicon is used for the semiconductor layer of thetransistor, simplification of a manufacturing process can be realized.

When the signal line driver circuit 1801, the scan line driver circuit1802, and the pixel portion 1804 are formed over the same substrate,polysilicon or single crystalline silicon is preferably used for thesemiconductor layers of the transistors included in the scan line drivercircuit 1802 and the pixels 1803.

The number, arrangement, and the like of the driver circuits are notlimited to those shown in FIG. 18 as long as pixels can be selected anda video signal can be independently written to each pixel as shown inFIG. 18.

For example, as shown in FIG. 20, the scan lines G1 to Gn may be scannedby a first scan line driver circuit 2002 a and a second scan line drivercircuit 2002 b. The first scan line driver circuit 2002 a and the secondscan line driver circuit 2002 b each have a structure similar to that ofthe scan line driver circuit 1802 shown in FIG. 18, and scan the scanlines G1 to Gn at the same timing. Further, the first scan line drivercircuit 2002 a and the second scan line driver circuit 2002 b may bereferred to as a first driver circuit and a second driver circuit.

Even if a defect is generated in one of the first scan line drivercircuit 2002 a and the second scan line driver circuit 2002 b, the scanlines G1 to Gn can be scanned by the other of the first scan line drivercircuit 2002 a and the second scan line driver circuit 2002 b; thus, thedisplay device of FIG. 20 can have redundancy. In the display device ofFIG. 20, a load (wiring resistance of the scan lines and parasiticcapacitance of the scan lines) of the first scan line driver circuit2002 a and a load of the second scan line driver circuit 2002 b can bereduced to half of that of FIG. 18. Thus, delay and distortion ofsignals input to the scan lines G1 to Gn (output signals of the firstscan line driver circuit 2002 a and the second scan line driver circuit2002 b) can be reduced. Further, since the loads of the first scan linedriver circuit 2002 a and the second scan line driver circuit 2002 b canbe reduced in the display device of FIG. 20, the scan lines G1 to Gn canbe scanned with high speed. Furthermore, since the scan lines G1 to Gncan be scanned with high speed, increase in size or definition of apanel can be realized. The merits of the display device of FIG. 20 aremore effective when amorphous silicon is used for the semiconductorlayers of transistors included in the first scan line driver circuit2002 a and the second scan line driver circuit 2002 b. Note thatcomponents in common with those of FIG. 18 are denoted by commonreference numerals, and the description is omitted.

As another example, FIG. 8 shows a display device in which a videosignal can be written to pixels with high speed. In the display deviceof FIG. 8, the same video signal is input to the pixel 1803 in the i-throw and j-th column and to the pixel 1803 in the (i+1)th row and (j+1)thcolumn. In the display device of FIG. 8, scan lines in odd-numberedstages among the scan lines G1 to Gn are scanned by a first scan linedriver circuit 802 a, and scan lines in even-numbered stages among thescan lines G1 to Gn are scanned by a second scan line driver circuit 802b. Further, the input of a start signal to the second scan line drivercircuit 802 b is delayed for ¼ cycle of a clock signal with respect tothe input of a start signal to the first scan line driver circuit 802 a.

The display device of FIG. 8 can perform dot inversion driving simply byinputting a video signal with positive polarity to every other signalline and inputting a video signal with negative polarity to the othersignal lines during one frame period. Further, the display device ofFIG. 8 can perform frame inversion driving by inverting polarity of thevideo signal input to each signal line in every frame period.

An operation of the display device of FIG. 8 is described with referenceto a timing chart of FIG. 9. The timing chart of FIG. 9 shows eachtiming for selecting the scan line G1 in the first row, the scan lineGi−1 in the (i−1)th row, the scan line Gi in the i-th row, the scan lineGi+1 in the (i+1)th row, and the scan line Gn in the n-th row. Further,in the timing chart of FIG. 9, one selection period is divided into aselection period a and a selection period b. The case where the displaydevice in FIG. 8 performs dot inversion driving and frame inversiondriving is described with reference to the timing chart of FIG. 9.

In FIG. 9, the selection period a of the scan line Gi in the i-th row,for example, overlaps with the selection period b of the scan line Gi−1in the (i−1)th row. The selection period b of the scan line Gi in thei-th row overlaps with the selection period a of the scan line Gi+1 inthe (i+1)th row. Therefore, in the selection period a, a video signalsimilar to that input to the pixel 1803 in the (i−1)th row and (j+1)thcolumn is input to the pixel 1803 in the i-th row and j-th column.Further, in the selection period b, a video signal similar to that inputto the pixel 1803 in the i-th row and j-th column is input to the pixel1803 in the (i+1)th row and (j+1)th column. Note that a video signalinput to the pixel 1803 in the selection period b is an original videosignal, and a video signal input to the pixel 1803 in the selectionperiod a is a video signal for precharging the pixel 1803. Accordingly,in the selection period a, each pixel 1803 is precharged by the videosignal input to the pixel 1803 in the (i−1)th row and (j+1)th column,and in the selection period b, an original video signal (in the i-th rowand j-th column) is input to each pixel 1803.

Accordingly, since the video signal can be written to the pixel 1803with high speed, increase in size or definition of the display device inFIG. 8 can be realized. Further, in the display device of FIG. 8, sincethe video signals with the same polarity are input to respective signallines in one frame period, the amount of charging and discharging ofeach signal line is decreased, and reduction in power consumption can berealized. Further, since a load of an IC for supplying the video signalcan be greatly decreased in the display device of FIG. 8, heatgeneration, power consumption, and the like of the IC can be reduced.Furthermore, driving frequency of the first scan line driver circuit 802a and the second scan line driver circuit 802 b in the display device ofFIG. 8 can be decreased to approximately half.

Note that, for the display device of this embodiment mode, variousdriving methods can be employed depending on a structure and a drivingmethod of the pixel 1803. For example, in one frame period, the scanline driver circuit may scan the scan lines a plurality of times.

An additional wiring or the like may be provided in the display devicesof FIGS. 8, 18, and 20 depending on a structure of the pixel 1803. Forexample, a power supply line maintained at a constant potential, acapacitor line, an additional scan line, or the like may be added. Whenan additional scan line is provided, an additional scan line drivercircuit to which the shift register of this embodiment mode is appliedmay be provided as well. As another example, the pixel portion may beprovided with a dummy scan line, signal line, power supply line, orcapacitor line.

Although this embodiment mode has been described with reference tovarious drawings, the contents (or part of the contents) described ineach drawing can be freely applied to, combined with, or replaced withthe contents (or part of the contents) described in another drawing.Further, much more drawings can be formed by combining each part in theabove-described drawings with another part.

Similarly, the contents (or part of the contents) described in eachdrawing in this embodiment mode can be freely applied to, combined with,or replaced with the contents (or part of the contents) described in adrawing in another embodiment mode. Further, much more drawings can beformed by combining each part in the drawings in this embodiment modewith part of another embodiment mode.

Note that this embodiment mode has described just examples of embodying,slightly transforming, modifying, improving, describing in detail, orapplying the contents (or part of the contents) described in otherembodiment modes, an example of related part thereof, or the like.Therefore, the contents described in other embodiment modes can befreely applied to, combined with, or replaced with the contentsdescribed in this embodiment mode.

Embodiment Mode 2

This embodiment mode describes structures and driving methods of aflip-flop different from those in Embodiment Mode 1, a driver circuitincluding the flip-flop, and a display device including the drivercircuit. Note that components in common with those in Embodiment Mode 1are denoted by common reference numerals, and detailed description ofthe same portions and portions having similar functions is omitted.

A flip-flop of this embodiment mode can have a structure similar to thatof the flip-flop in Embodiment Mode 1. Thus, in this embodiment mode,description of the structure of the flip-flop is omitted. Note thattiming for driving the flip-flop is different from that in EmbodimentMode 1.

The case where driving timing of this embodiment mode is applied to theflip-flop in FIG. 1 is described. The driving timing of this embodimentmode can be freely combined with each flip-flop in FIGS. 4A to 4D, 5A to5D, 7A to 7C, and 21A to 21C as well. Further, the driving timing ofthis embodiment mode can be freely combined with the driving timing ofEmbodiment Mode 1 as well.

An operation of the flip-flop of this embodiment mode is described withreference to the flip-flop in FIG. 1 and a timing chart of FIG. 31. Thetiming chart of FIG. 31 is described with an operation period dividedinto a selection period and a non-selection period. Further, thenon-selection period is divided into a first non-selection period, asecond non-selection period, a set period A, a set period A′, and areset period. Furthermore, the selection period is divided into aselection period B and a selection period B′. During an operation periodin the non-selection period except for the set period A, the set periodA′, the selection period B, the selection period B′, and the resetperiod, the first non-selection period and the second non-selectionperiod are sequentially repeated.

Note that in FIG. 31, a signal 3121, a signal 3125, a signal 3126, apotential 3141, a potential 3142, a signal 3122, and a signal 3123 referto a signal input to the first wiring 121, a signal input to the fifthwiring 125, a signal input to the sixth wiring 126, a potential of thenode 141, a potential of the node 142, a signal input to the secondwiring 122, and a signal output through the third wiring 123,respectively.

The signal 3121, the signal 3125, the signal 3126, the potential 3141,the potential 3142, the signal 3122, and the signal 3123 correspond tothe signal 221, the signal 225, the signal 226, the potential 241, thepotential 242, the signal 222, and the signal 223 shown in FIG. 2,respectively and have similar characteristics.

The flip-flop of this embodiment mode basically operates similarly tothe flip-flop of Embodiment Mode 1. The flip-flop of this embodimentmode is different from the flip-flop of Embodiment Mode 1 in that timingat which the H signal is input to the first wiring 121 is delayed for ¼cycle of a clock signal.

The operation of the flip-flop of this embodiment mode in the firstnon-selection period and the second non-selection period is similar tothat of the flip-flop of Embodiment Mode 1 in the first non-selectionperiod and the second non-selection period. The operation of theflip-flop of this embodiment mode in the set period A is similar to thatin the second non-selection period. The operation of the flip-flop ofthis embodiment mode in the reset period is similar to that of theflip-flop of Embodiment Mode 1 in the reset period. The operation of theflip-flop of this embodiment mode in the selection period B and theselection period B′ is similar to that of the flip-flop of EmbodimentMode 1 in the selection period. Note that the flip-flop of thisembodiment mode is different from the flip-flop of Embodiment Mode 1 inthat the H signal is input to the first wiring 121 in the selectionperiod B. However, the input of the H signal to the first wiring 121 inthe selection period B hardly affects the operation in this embodimentmode because the fifth transistor 105 remains off. Thus, the detaileddescription of the flip-flop of this embodiment mode in the set periodA, the set period A′, the selection period B, the selection period B′,the reset period, the first non-selection period, and the secondnon-selection period is omitted.

The flip-flop of this embodiment mode can obtain advantageous effectssimilar to those of the flip-flop of Embodiment Mode 1.

Note that, by application of a timing chart shown in FIG. 32 to theflip-flop of this embodiment mode, fall time of output signal of theflip-flop of this embodiment mode can be shortened significantly. Thisis because the L signal can be input to the third wiring 123 through thefirst transistor 101 by delaying timing at which the signal 3122 (resetsignal) becomes the H level.

Next, a structure and a driving method of a shift register including theaforementioned flip-flop of this embodiment mode are described.

A structure of a shift register of this embodiment mode is describedwith reference to FIG. 33. The shift register of FIG. 33 includes nflip-flops (flip-flops 3301_1 to 3301 _(—) n).

The connection relationship of the shift register in FIG. 33 isdescribed. Of the flip-flops_(—) i in the i-th row (the flip-flops3301_1 to 3301 _(—) n) in the shift register of FIG. 33, the flip-flop3301_4N−3 in the (i=4N−3)th stage (N is a natural number equal to orgreater than 2) and the flip-flop 3301_4N−1 in the (i=4N−1)th stage (Nis a natural number equal to or greater than 1) are connected to asecond wiring 3312, a fourth wiring 3314, a sixth wiring 3316, a seventhwiring 3317, an eighth wiring 3318, an eleventh wiring 3321 _(—) i−1, aneleventh wiring 3321 _(—) i, and an eleventh wiring 3321 _(—) i+2. Notethat the flip-flop 3301_1 in the (i=4N−3)th stage (N=1) is connected toa first wiring 3111, the second wiring 3312, the fourth wiring 3314, thesixth wiring 3316, the seventh wiring 33317, the eighth wiring 3318, aneleventh wiring 3321_1, and an eleventh wiring 3321_3. Further, theflip-flop 33014N−2 in the (i=4N−2)th stage (N is a natural number equalto or greater than 1) and the flip-flop 3301_4N in the (i=4N)th stage (Nis a natural number equal to or greater than 1) are connected to a thirdwiring 3313, a fifth wiring 3315, the sixth wiring 3316, the seventhwiring 3317, the eighth wiring 3318, the eleventh wiring 3321 _(—) i−1,the eleventh wiring 3321 _(—) i, and the eleventh wiring 3321 _(—) i+2.The flip-flop 3301 _(—) n−1 in the (n−1)th stage is connected to thesecond wiring 3312, the fourth wiring 3314, the sixth wiring 3316, theseventh wiring 3317, the eighth wiring 3318, a tenth wiring 3320, aneleventh wiring 3321 _(—) n−2, and an eleventh wiring 3321 _(—) n−1. Theflip-flop 3301 _(—) n in the n-th stage is connected to the third wiring3313, the fifth wiring 3315, the sixth wiring 3316, the seventh wiring3317, the eighth wiring 3318, a ninth wiring 3319, the eleventh wiring3321 _(—) n−1, and an eleventh wiring 3321 _(—) n.

The first wiring 3311 is connected to the first wiring 121 shown in FIG.1 of the flip-flop 3301_1. The second wiring 3312 is connected to thefifth wiring 125 shown in FIG. 1 of the flip-flop 3301_4N−3 and isconnected to the sixth wiring 126 shown in FIG. 1 of the flip-flop3301_4N−1. The third wiring 3313 is connected to the fifth wiring 125shown in FIG. 1 of the flip-flop 3301_4N−2 and is connected to the sixthwiring 126 shown in FIG. 1 of the flip-flop 3301_4N. The fourth wiring3314 is connected to the sixth wiring 126 shown in FIG. 1 of theflip-flop 3301_4N−3 and is connected to the fifth wiring 125 shown inFIG. 1 of the flip-flop 33014N−1. The fifth wiring 3315 is connected tothe sixth wiring 126 shown in FIG. 1 of the flip-flop 3301_4N−2 and isconnected to the fifth wiring 125 shown in FIG. 1 of the flip-flop3301_4N. The sixth wiring 3316 is connected to the seventh wiring 127shown in FIG. 1 of the flip-flops in all stages. The seventh wiring 3317is connected to the eighth wiring 128 shown in FIG. 1 of the flip-flopsin all stages. The eighth wiring 3318 is connected to the fourth wiring124, the ninth wiring 129, the tenth wiring 130, and the eleventh wiring131 shown in FIG. 1 of the flip-flops in all stages. The ninth wiring3319 is connected to the second wiring 122 shown in FIG. 1 of theflip-flop 3301 _(—) n. The tenth wiring 3320 is connected to the secondwiring 122 shown in FIG. 1 of the flip-flop 3301 _(—) n−1. The eleventhwiring 3321 _(—) i is connected to the second wiring 122 shown in FIG. 1of the flip-flop 3301 _(—) i−2, the third wiring 123 shown in FIG. 1 ofthe flip-flop 3301 _(—) i, and the first wiring 121 shown in FIG. 1 ofthe flip-flop 3301 _(—) i+1. Note that the eleventh wiring 3321_1 isconnected to the third wiring 123 shown in FIG. 1 of the flip-flop3301_1 and the first wiring 121 shown in FIG. 1 of the flip-flop 3301_2.The eleventh wiring 3321_2 is connected to the third wiring 123 shown inFIG. 1 of the flip-flop 3301_2 and the first wiring 121 shown in FIG. 1of the flip-flop 3301_3. The eleventh wiring 3321 _(—) n is connected tothe third wiring 123 shown in FIG. 1 of the flip-flop 3301 _(—) n.

Note that the sixth wiring 3316 and the seventh wiring 3317 are eachsupplied with the potential V1, and the eighth wiring 3318 is suppliedwith the potential V2.

Note that signals are input to the first wiring 3311, the second wiring3312, the third wiring 3313, the fourth wiring 3314, the fifth wiring3315, the ninth wiring 3319, and the tenth wiring 3320. The signal inputto the first wiring 3311 is a start signal; the signal input to thesecond wiring 3312 is a first clock signal; the signal input to thethird wiring 3313 is a second clock signal; the signal input to thefourth wiring 3314 is a third clock signal; the signal input to thefifth wiring 3315 is a fourth clock signal; the signal input to theninth wiring 3319 is a first reset signal; and the signal input to thetenth wiring 3320 is a second reset signal. In addition, each of thesignals input to the first wiring 3311, the second wiring 3312, thethird wiring 3313, the fourth wiring 3314, the fifth wiring 3315, theninth wiring 3319, and the tenth wiring 3320 is a digital signalincluding an H signal with a potential of V1 and an L signal with apotential of V2.

Various signals, currents, or voltages may be input to the first totenth wirings 3311 to 3320.

Signals are output through the eleventh wirings 3321_1 to 3321 _(—) n.For example, the signal output through the eleventh wiring 3321 _(—) iis an output signal of the flip-flop 3301 _(—) i. Further, the signaloutput through the eleventh wiring 3321 _(—) i is an input signal of theflip-flop 3301 _(—) i+1 and a reset signal of the flip-flop 3301 _(—)i−2.

Next, the operation of the shift register shown in FIG. 33 is describedwith reference to a timing chart of FIG. 35 and a timing chart of FIG.36. The timing chart of FIG. 35 is divided into a scan period and aretrace period. The scan period corresponds to a period from the timewhen output of a selection signal through the eleventh wiring 3311_1starts to the time when output of a selection signal through theeleventh wiring 3311 _(—) n ends. The retrace period corresponds to aperiod from the time when output of the selection signal through theeleventh wiring 3311 _(—) n ends to the time when output of theselection signal through the eleventh wiring 3311_1 starts.

Note that FIG. 35 shows a signal 3511 input to the first wiring 3311, asignal 3512 input to the second wiring 3312, a signal 3513 input to thethird wiring 3313, a signal 3514 input to the fourth wiring 3314, asignal 3515 input to the fifth wiring 3315, a signal 3519 input to theninth wiring 3319, a signal 3520 input to the tenth wiring 3320, asignal 3521_1 output to the eleventh wiring 3321_1, and a signal 3521_(—) n output to the eleventh wiring 3321 _(—) n. FIG. 36 shows a signal3611 input to the first wiring 3311, a signal 3621_1 output to theeleventh wiring 3321_1, a signal 3621 _(—) i−1 output to the eleventhwiring 3321 _(—) i−1, a signal 3621 _(—) i output to the eleventh wiring3321 _(—) i, a signal 3621 _(—) i+1 output to the eleventh wiring 3321_(—) i+1, and a signal 3621 _(—) n output to the eleventh wiring 3321_(—) n.

As shown in FIG. 36, if the flip-flop 3301 _(—) i−1, for example, is inthe selection period B, the H signal is output through the eleventhwiring 3321 _(—) i−1. At this time, the flip-flop 3301 _(—) i is in theset period A′. Subsequently, the flip-flop 3301 _(—) i−1 is in theselection period B′, and the H signal is output through the eleventhwiring 3321 _(—) i−1. At this time, the flip-flop 3301 _(—) i is in theselection period B. After that, the flip-flop 3301 _(—) i−1 is in thereset period, and the L signal is output through the eleventh wiring3321 _(—) i−1. At this time, the flip-flop 3301 _(—) i is in theselection period B′. In other words, in the shift register of thisembodiment mode, the H signal is output sequentially from the flip-flop3301 _(—) i−1, and there is a period in which the selection period B′ ofthe flip-flop 3301 _(—) i−1 overlaps with the selection period B of theflip-flop 3301 _(—) i.

Note that when the timing chart of FIG. 32 is applied to the flip-flopof this specification, the shift register may have a structure as shownin FIG. 34. In the shift register of FIG. 34, the second wiring 122shown in FIG. 1 of the flip-flop 3301 _(—) i in the i-th stage isconnected to an eleventh wiring 3321 _(—) i+3. The second wiring 122shown in FIG. 1 of the flip-flop 3301 _(—) n−2 is connected to a twelfthwiring 3322 to which a third reset signal is input. Note that componentsin common with FIG. 33 are denoted by common reference numerals, and thedescription is omitted.

The shift register of this embodiment mode, to which the flip-flop ofthis embodiment mode is applied, can achieve suppression of a shift inthreshold voltage of a transistor, extension of life, improvement ofdrive capability, suppression of malfunction, simplification of aprocess, and the like.

The shift register of this embodiment mode can be freely combined withthe shift register described in Embodiment Mode 1. For example, theshift register of this embodiment mode can be freely combined with theshift register of FIG. 13, 14, 15, or 17. Specifically, in the shiftregister of this embodiment mode, buffers may be connected to theeleventh wirings 3321_1 to 3321 _(—) n, a reset signal may be generatedinside, or a dummy flip-flop may be provided. As mentioned above,components in common with Embodiment Mode 1 are denoted by commonreference numerals, and the description is omitted.

Next, a structure and a driving method of a display device including theaforementioned shift register of this embodiment mode are described.Note that a display device of this embodiment mode includes at least theflip-flop of this embodiment mode.

A structure of the display device of this embodiment mode is describedwith reference to FIG. 16. In the display device of FIG. 16, the scanlines G1 to Gn are scanned by a scan line driver circuit 1602. Further,in the display device of FIG. 16, the same video signal is input to thepixel 1803 in the i-th row and j-th column and to the pixel 1803 in the(i+1)th row and (j+1)th column. Note that components in common withthose in FIG. 18 are denoted by common reference numerals, and thedescription is omitted.

When the shift register of this embodiment mode is applied to the scanline driver circuit 1602, the display device of FIG. 16 can operatesimilarly to the display device of FIG. 8 with one scan line drivercircuit. Thus, the display device of FIG. 16 can provide advantageouseffects similar to the display device of FIG. 8.

Similarly to FIG. 20, the scan lines G1 to Gn may be scanned by a firstscan line driver circuit 2202 a and a second scan line driver circuit2202 b. Thus, advantageous effects similar to those of the displaydevice of FIG. 20 can be obtained. FIG. 22 shows a structure in thiscase.

Although this embodiment mode has been described with reference tovarious drawings, the contents (or part of the contents) described ineach drawing can be freely applied to, combined with, or replaced withthe contents (or part of the contents) described in another drawing.Further, much more drawings can be formed by combining each part in theabove-described drawings with another part.

Similarly, the contents (or part of the contents) described in eachdrawing in this embodiment mode can be freely applied to, combined with,or replaced with the contents (or part of the contents) described in adrawing in another embodiment mode. Further, much more drawings can beformed by combining each part in the drawings in this embodiment modewith part of another embodiment mode.

Note that this embodiment mode has described just examples of embodying,slightly transforming, modifying, improving, describing in detail, orapplying the contents (or part of the contents) described in otherembodiment modes, an example of related part thereof, or the like.Therefore, the contents described in other embodiment modes can befreely applied to, combined with, or replaced with the contentsdescribed in this embodiment mode.

Embodiment Mode 3

This embodiment mode describes structures and driving methods of aflip-flop different from those in Embodiment Modes 1 and 2, a drivercircuit including the flip-flop, and a display device including thedriver circuit. In the flip-flop of this embodiment mode, an outputsignal and a transfer signal of the flip-flop are output throughdifferent wirings by different transistors. Note that components incommon with those of Embodiment Modes 1 and 2 are denoted by commonreference numerals, and detailed description of the same portions andportions having similar functions is omitted.

A basic structure of the flip-flop of this embodiment mode is describedwith reference to FIG. 27. The flip-flop in FIG. 27 is similar to theflip-flop in FIG. 1 to which an eighth transistor 108 and a ninthtransistor 109 are added.

The connection relationship of the flip-flop in FIG. 27 is described. Afirst electrode of the eighth transistor 108 is connected to athirteenth wiring 133, a second electrode of the eighth transistor 108is connected to a twelfth wiring 132, and a gate electrode of the eighthtransistor 108 is connected to the node 141. A first electrode of theninth transistor 109 is connected to a fourteenth wiring 134, a secondelectrode of the ninth transistor 109 is connected to the twelfth wiring132, and a gate electrode of the ninth transistor 109 is connected tothe node 142. The other connection relationship is similar to FIG. 1.

The twelfth wiring 132 and the thirteenth wiring 133 may be referred toas a sixth signal line and a seventh signal line, respectively. Thefourteenth wiring 134 may be referred to as a seventh power supply line.

Note that the fourteenth wiring 134 is supplied with V2.

A signal is input to the thirteenth wiring 133. The signal input to thethirteenth wiring 133 may be a similar signal to that input to the fifthwiring 125.

A signal is output through the twelfth wiring 132. As described inEmbodiment Mode 1, a signal is also output through the third wiring 123.

Note that signals input to or potentials supplied to the first wiring121, the second wiring 122, the fourth wiring 124, the fifth wiring 125,the sixth wiring 126, the seventh wiring 127, the eighth wiring 128, theninth wiring 129, the tenth wiring 130, and the eleventh wiring 131 aresimilar to those of FIG. 1.

The flip-flop of FIG. 27 is described as the flip-flop in FIG. 1 towhich the eighth transistor 108 and the ninth transistor 109 are added;however, the eighth transistor 108 and the ninth flip-flop 109 may beadded to the flip-flop shown in FIGS. 4A to 4D, 5A to 5D, 7A to 7C, and21A to 21C.

Next, an operation of the flip-flop shown in FIG. 27 is described withreference to a timing chart of FIG. 28. Portions in common with those inthe timing chart of FIG. 2 are denoted by common reference numeral, andthe description is omitted.

Note that a signal 232 refers to the signal output through the twelfthwiring 132. The signal 221, the signal 225, the signal 226, thepotential 241, the potential 242, the signal 222, and the signal 223 aresimilar to those in FIG. 2. Note that the signal 221, the signal 225,the signal 226, the potential 241, the potential 242, the signal 222,and the signal 223 may be similar to those in FIG. 6, 31, or 32.

In this embodiment mode, as described above, an output signal and atransfer signal of the flip-flop are output through different wirings bydifferent transistors. In other words, in the flip-flop of FIG. 27, asignal is output through the third wiring 123 by the first transistor101 and the second transistor 102, and a signal is output through thetwelfth wiring 132 by the eighth transistor 108 and the ninth transistor109. The eighth transistor 108 and the ninth transistor 109 areconnected in the same manner as the first transistor 101 and the secondtransistor 102; thus, as shown in FIG. 28, the signal output through thetwelfth wiring 132 (the signal 232) has approximately the same waveformas the signal output through the third wiring 123 (the signal 223).Here, the signal 232 is the output signal of the flip-flop, and thesignal 223 is the transfer signal of the flip-flop. Note that the signal223 may be used as the output signal of the flip-flop, and the signal232 may be used as the transfer signal of the flip-flop.

The eighth transistor 108 and the ninth transistor 109 have functionssimilar to those of the first transistor 101 and the second transistor102, respectively. Further, the eighth transistor 108 and the ninthtransistor 109 may be referred to as a buffer portion.

Accordingly, even when a large load is connected to the twelfth wiring132 and the signal 232 is delayed or distorted, malfunction of theflip-flop in FIG. 27 can be prevented. This is because the delay,distortion, or the like of the output signal does not affect theoperation of the flip-flop in FIG. 27 when the output signal and thetransfer signal of the flip-flop are output through different wirings bydifferent transistors.

The flip-flop of this embodiment mode can provide advantageous effectssimilar to those of the flip-flops in Embodiment Modes 1 and 2.

Note that the operation timing described in Embodiment Mode 2 can beapplied to the flip-flop of this embodiment mode.

A structure and a driving method of a shift register including theaforementioned flip-flop of this embodiment mode are described.

A structure of the shift register of this embodiment mode is describedwith reference to FIG. 29. The shift register of FIG. 29 includes nflip-flops (flip-flops 2901_1 to 2901 _(—) n).

The flip-flops 2901_1 to 2901 _(—) n, a first wiring 2911, a secondwiring 2912, a third wiring 2913, a fourth wiring 2914, a fifth wiring2915, a sixth wiring 2916, and a seventh wiring 2917 correspond to theflip-flops 1001_1 to 1001 _(—) n, the first wiring 1011, the secondwiring 1012, the third wiring 1013, the fourth wiring 1014, the fifthwiring 1015, the sixth wiring 1016, and the seventh wiring 1017 of FIG.10, respectively and similar signals or power supply voltages are inputthereto. Eighth wirings 2918_1 to 2918 _(—) n and ninth wiring 2919_1 to2919 _(—) n correspond to the tenth wirings 1018_1 to 1018 _(—) n.

Next, an operation of the shift register in FIG. 29 is described withreference to a timing chart of FIG. 30.

The operation of the shift register in FIG. 29 is different from that ofthe shift register in FIG. 10 in that the output signal and the transfersignal are output through different wirings. Specifically, the outputsignals are output through the eighth wirings 2918_1 to 2918 _(—) n, andthe transfer signals are output through the ninth wirings 2919_1 to 2919_(—) n.

Even when a large load (e.g., resistor or capacitor) is connected to theninth wirings 2919_1 to 2919 _(—) n, the shift register in FIG. 29 canoperate without being affected by the load. Further, even when any ofthe ninth wirings 2919_1 to 2919 _(—) n is short-circuited with thepower supply line or the signal line, the shift register in FIG. 29 cancontinue to operate normally. Accordingly, the shift register of FIG. 29can achieve improvement of drive capability. This is because thetransfer signal and the output signal of each flip-flop are separated inthe shift register in FIG. 29.

Further, the shift register of FIG. 29 can obtain merits such asreduction in layout area, suppression of a shift in threshold voltage ofa transistor, simplification of a process, manufacturing of asemiconductor device like a large-scale display device, andmanufacturing of a semiconductor device like a long-life display panel,by employing the flip-flop of this embodiment mode.

Note that the shift register is not limited to the structure of FIG. 29if a similar operation to FIG. 29 is achieved. For example, bycombination with the shift register of FIG. 13, 14, 15, or 17, a similarmerit to FIG. 13, 14, 15, or 17 can be obtained.

A structure and a driving method of a display device including theaforementioned shift register of this embodiment mode are described.Note that the display device of this embodiment mode includes at leastthe flip-flop of this embodiment mode.

As the display device of this embodiment mode, the display device ofFIGS. 8, 16, 18, 20, or 22 can be used. Thus, the display device of thisembodiment mode can obtain advantageous effects similar to the displaydevices described in Embodiment Modes 1 and 2.

Although this embodiment mode has been described with reference tovarious drawings, the contents (or part of the contents) described ineach drawing can be freely applied to, combined with, or replaced withthe contents (or part of the contents) described in another drawing.Further, much more drawings can be formed by combining each part in theabove-described drawings with another part.

Similarly, the contents (or part of the contents) described in eachdrawing in this embodiment mode can be freely applied to, combined with,or replaced with the contents (or part of the contents) described in adrawing in another embodiment mode. Further, much more drawings can beformed by combining each part in the drawings in this embodiment modewith part of another embodiment mode.

Note that this embodiment mode has described just examples of embodying,slightly transforming, modifying, improving, describing in detail, orapplying the contents (or part of the contents) described in otherembodiment modes, an example of related part thereof, or the like.Therefore, the contents described in other embodiment modes can befreely applied to, combined with, or replaced with the contentsdescribed in this embodiment mode.

Embodiment Mode 4

This embodiment mode describes a case where a p-channel transistor isused as each transistor included in a flip-flop of this specification.In addition, it describes structures and driving methods of a drivercircuit including the flip-flop and a display device including thedriver circuit.

As the flip-flop of this embodiment mode, the case where the polarity ofeach transistor included in the flip-flop of FIG. 1 is changed top-channel type is described. Thus, the flip-flop of this embodiment modecan obtain advantageous effects similar to those of the flip-flop inFIG. 1. Note that the polarity of each transistor included in eachflip-flop shown in FIGS. 4A to 4D, 5A to 5D, 7A to 7C, 21A to 21C, and27 may be changed to p-channel type. Note also that the flip-flop ofthis embodiment mode can be freely combined with the description inEmbodiment Modes 1 to 3.

A basic structure of the flip-flop of this embodiment mode is describedwith reference to FIG. 23. The flip-flop shown in FIG. 23 includes firstto seventh transistors 2301 to 2307. The first to seventh transistors2301 to 2307 correspond to the first to seventh transistors 101 to 107in FIG. 1, respectively. Note that each of the first to seventhtransistors 2301 to 2307 is a p-channel transistor and becomesconductive when the absolute value of a gate-source voltage (|Vgs|)exceeds the absolute value of a threshold voltage (|Vth|) (when Vgs isbelow Vth).

Note that the connection relationship of the flip-flop in FIG. 23 issimilar to that in FIG. 1, so that the description is omitted.

Note that a connection portion of a gate electrode of the firsttransistor 2301, a gate electrode of the fourth transistor 2304, asecond electrode of the fifth transistor 2305, a second electrode of thesixth transistor 2306, and a second electrode of the seventh transistor2307 is referred to as a node 2341. A connection portion of a gateelectrode of the second transistor 2302, a second electrode of the thirdtransistor 2303, a second electrode of the fourth transistor 2304, and agate electrode of the sixth transistor 2306 is referred to as a node2342.

A fourth wiring 2324, a ninth wiring 2329, a tenth wiring 2330, and aneleventh wiring 2331 may be connected to each other or may be a singlewiring. A seventh wiring 2327 and an eighth wiring 2328 may be connectedto each other or may be a single wiring.

The fourth wiring 2324, the seventh wiring 2327, the eighth wiring 2328,the ninth wiring 2329, the tenth wiring 2330, and the eleventh wiring2331 correspond to the fourth wiring 124, the seventh wiring 127, theeighth wiring 128, the ninth wiring 129, the tenth wiring 130, and theeleventh wiring 131 of FIG. 1, respectively. First to third wirings 2321to 2323 and fifth and sixth wirings 2325 and 2326 correspond to thefirst to third wirings 121 to 123 and the fifth and sixth wirings 125and 126 of FIG. 1, respectively. Note that the H level and the L levelof signals input to, potentials supplied to, or a signal output throughthe first to eleventh wirings 2321 to 2331 are reversed with respect tothe signals input to, potentials supplied to, or signal output throughthe first to eleventh wirings 121 to 131 of FIG. 1.

The seventh wiring 2327 and the eighth wiring 2328 are each suppliedwith the potential V2, and the fourth wiring 2324, the ninth wiring2329, the tenth wiring 2330, and the eleventh wiring 2331 are eachsupplied with the potential V1.

Next, an operation of the flip-flop shown in FIG. 23 is described withreference to a timing chart of FIG. 24.

Note that the timing chart of FIG. 24 is similar to a timing chart withthe H level and the L level reversed with respect to the timing chart ofFIG. 2. A signal 2421, a signal 2425, a signal 2425, a potential 2441, apotential 2442, a signal 2422, and a signal 2423 correspond to thesignal 221, the signal 225, the signal 226, the potential 241, thepotential 242, the signal 222, and the signal 223 of FIG. 2.

Note that not only the timing chart with the H level and the L levelreversed with respect to FIG. 2 but also timing charts with the H leveland the L level reversed with respect to FIGS. 6, 28, 31, and 32 may beapplied to the flip-flop of this embodiment mode.

First, an operation of the flip-flop in a set period denoted by (A) inFIG. 24 is described. The potential 2441 of the node 2341 isV2+|Vth23051 (Vth2305: a threshold voltage of the fifth transistor2305). The node 2341 is in a floating state with its potential 2441maintained at V2+|Vth23051. At this time, the potential 2442 of the node2342 is V1−θ (θ: a given positive number). Note that since the firsttransistor 2301 and the second transistor 2302 are turned on, the Hsignal is output through the third wiring 2323.

An operation of the flip-flop in a selection period denoted by (B) inFIG. 24 is described. The potential 2441 of the node 2341 becomesV2−|Vth2301|−γ (Vth2301: a threshold voltage of the first transistor2301 and γ: a given positive number). Thus, the first transistor 2301 isturned on, and the L signal is output through the third wiring 2323. Atthis time, the potential 2442 of the second node 2342 becomes V1.

An operation of the flip-flop in a reset period denoted by (C) in FIG.24 is described. The seventh transistor 2307 is turned on, so that thepotential 2441 of the node 2341 becomes V1. Thus, the first transistor2301 is turned off. At this time, the second transistor 2302 is turnedon, so that the H signal is output through the third wiring 2323.

An operation of the flip-flop in a first non-selection period denoted by(D) in FIG. 24 is described. The potential 2442 of the node 2342 becomesV1, so that the second transistor 2302 and the sixth transistor 2306 areturned off. At this time, the node 2341 is in a floating state, so thatthe potential 2441 is maintained at V1.

An operation of the flip-flop in a second non-selection period denotedby (E) in FIG. 24 is described. The potential 2442 of the node 2342becomes V2+|Vth23031, so that the second transistor 2302 and the sixthtransistor 2306 are turned on. Therefore, the node 2341 and the thirdwiring 2323 are supplied with V1.

Furthermore, the flip-flop of FIG. 23 can suppress shifts in thresholdvoltage of the second transistor 2302 and the sixth transistor 2306because the second transistor 2302 and the sixth transistor 2306 areturned on only in the second non-selection period.

Note that the flip-flop of FIG. 23 can also suppress a shift inthreshold voltage of the third transistor 2303 by supplying V2 to thegate electrode of the third transistor 2303 and inputting the secondclock signal to the first electrode.

In addition, the flip-flop of FIG. 23 can suppress shifts in thresholdvoltage of the first transistor 2301, the fourth transistor 2304, thefifth transistor 2305, and the seventh transistor 2307 because the firsttransistor 2301, the fourth transistor 2304, the fifth transistor 2305,and the seventh transistor 2307 are not turned on in the firstnon-selection period and the second non-selection period.

Further, the flip-flop of FIG. 23 can reset the potential 2441 of thenode 2341 and the potential of the third wiring 2323 to V1 by supplyingV1 to the node 2341 and the third wiring 2323 in the secondnon-selection period even if the potential 2441 of the node 2341 and thepotential of the third wiring 2323 fluctuate in the first non-selectionperiod. Thus, the flip-flop of FIG. 23 can suppress malfunction of whichcause is that the node 2341 and the third wiring 2323 are in a floatingstate and the potential 2441 of the node 2341 and the potential of thethird wiring 2323 fluctuate.

Furthermore, because the flip-flop of FIG. 23 can suppress a shift inthreshold voltage of a transistor, the flip-flop of FIG. 23 can suppressmalfunction of which cause is a shift in threshold voltage of atransistor.

Moreover, in the flip-flop of FIG. 23, all of the first to seventhtransistors 2301 to 2307 are p-channel transistors. Thus, the flip-flopof FIG. 23 can achieve simplification of a manufacturing process,reduction in manufacturing cost, and improvement in yield.

The arrangement, the number, and the like of the transistors are notlimited to those in FIG. 23 as long as an operation similar to FIG. 23is achieved. Thus, the flip-flop of FIG. 23 may be additionally providedwith a transistor, another element (such as a resistor or a capacitor),a diode, a switch, various logic circuits, or the like.

Note that the shift register of this embodiment mode can be embodied byfree combination of the flip-flop of this embodiment mode with any ofthe shift registers described in Embodiment Modes 1 to 3. For example,the shift register of this embodiment mode can be embodied by freecombination of the flip-flop of this embodiment mode with any of theshift register of FIGS. 10, 13, 14, 15, 17, 29, 33, and 34. Note thatthe H level and the L level of the shift register of this embodimentmode are reversed with respect to the shift registers described inEmbodiment Modes 1 to 3.

Note that a display device of this embodiment mode can be embodied byfree combination of the shift register of this embodiment mode with anyof the display devices described in Embodiment Modes 1 to 3. Forexample, the display device of this embodiment mode can be embodied byfree combination of the shift register of this embodiment mode with anyof the display devices of FIGS. 8, 16, 18, 20, and 22. Note that the Hlevel and the L level of the display device of this embodiment mode arereversed with respect to the display devices described in EmbodimentModes 1 to 3.

Although this embodiment mode has been described with reference tovarious drawings, the contents (or part of the contents) described ineach drawing can be freely applied to, combined with, or replaced withthe contents (or part of the contents) described in another drawing.Further, much more drawings can be formed by combining each part in theabove-described drawings with another part.

Similarly, the contents (or part of the contents) described in eachdrawing in this embodiment mode can be freely applied to, combined with,or replaced with the contents (or part of the contents) described in adrawing in another embodiment mode. Further, much more drawings can beformed by combining each part in the drawings in this embodiment modewith part of another embodiment mode.

Note that this embodiment mode has described just examples of embodying,slightly transforming, modifying, improving, describing in detail, orapplying the contents (or part of the contents) described in otherembodiment modes, an example of related part thereof, or the like.Therefore, the contents described in other embodiment modes can befreely applied to, combined with, or replaced with the contentsdescribed in this embodiment mode.

Embodiment Mode 5

This embodiment mode describes a signal line driver circuit included ineach display device described in Embodiment Modes 1 to 4.

A signal line driver circuit in FIG. 37 is described. The signal linedriver circuit in FIG. 37 includes a driver IC 5601, switch groups5602_1 to 5602_M, a first wiring 5611, a second wiring 5612, a thirdwiring 5613, and wirings 5621_1 to 5621_M. Each of the switch groups5602_1 to 5602_M includes a first switch 5603 a, a second switch 5603 b,and a third switch 5603 c.

The driver IC 5601 is connected to the first wiring 5611, the secondwiring 5612, the third wiring 5613, and the wirings 5621_1 to 5621_M.Each of the switch groups 5602_1 to 5602_M is connected to the firstwiring 5611, the second wiring 5612, the third wiring 5613, and one ofthe wirings 5621_1 to 5621_M corresponding to the switch groups 5602_1to 5602_M respectively. Each of the wirings 5621_1 to 5621_M isconnected to three signal lines through the first switch 5603 a, thesecond switch 5603 b, and the third switch 5603 c. For example, thewiring 5621_J in the J-th column (one of the wirings 5621_1 to 5621_M)is connected to a signal line Sj−1, a signal line Sj, and a signal lineSj+1 through the first switch 5603 a, the second switch 5603 b, and thethird switch 5603 c included in the switch group 5602_J.

Note that a signal is input to each of the first wiring 5611, the secondwiring 5612, and the third wiring 5613.

The driver IC 5601 is preferably formed using a single crystallinesubstrate or a glass substrate using a polycrystalline semiconductor.The switch groups 5602_1 to 5602_M are preferably formed over the samesubstrate as the pixel portion shown in Embodiment Mode 1. Therefore,the driver IC 5601 and the switch groups 5602_1 to 5602_M are preferablyconnected through an FPC or the like.

Next, an operation of the signal line driver circuit in FIG. 37 isdescribed with reference to a timing chart of FIG. 38. The timing chartof FIG. 38 shows a case where a scan line Gi in the i-th row isselected. A selection period of the scan line Gi in the i-th row isdivided into a first sub-selection period T1, a second sub-selectionperiod T2, and a third sub-selection period T3. The signal line drivercircuit in FIG. 37 operates similarly to FIG. 38 even when a scan linein another row is selected.

The timing chart of FIG. 38 shows a case where the wiring 5621_J in theJ-th column is connected to the signal line Sj−1, the signal line Sj,and the signal line Sj+1 through the first switch 5603 a, the secondswitch 5603 b, and the third switch 5603 c.

The timing chart of FIG. 38 shows timing at which the scan line Gi inthe i-th row is selected, timing 5703 a at which the first switch 5603 ais turned on or off, timing 5703 b at which the second switch 5603 b isturned on or off, timing 5703 c at which the third switch 5603 c isturned on or off, and a signal 5721_J input to the wiring 5621_J in theJ-th column.

In the first sub-selection period T1, the second sub-selection periodT2, and the third sub-selection period T3, different video signals areinput to the wirings 5621_1 to 5621_M. For example, a video signal inputto the wiring 5621_J in the first sub-selection period T1 is input tothe signal line Sj−1; a video signal input to the wiring 5621_J in thesecond sub-selection period T2 is input to the signal line Sj, and avideo signal input to the wiring 5621_J in the third sub-selectionperiod T3 is input to the signal line Sj+1. The video signals input tothe wiring 5621_J in the first sub-selection period T1, the secondsub-selection period T2, and the third sub-selection period T3 aredenoted by Dataj−1, Dataj, and Dataj+1, respectively.

As shown in FIG. 38, in the first sub-selection period T1, the firstswitch 5603 a is turned on, and the second switch 5603 b and the thirdswitch 5603 c are turned off. At this time, Dataj−1 input to the wiring5621_J is input to the signal line Sj−1 through the first switch 5603 a.In the second sub-selection period T2, the second switch 5603 b isturned on, and the first switch 5603 a and the third switch 5603 c areturned off. At this time, Dataj input to the wiring 5621J is input tothe signal line Sj through the second switch 5603 b. In the thirdsub-selection period T3, the third switch 5603 c is turned on, and thefirst switch 5603 a and the second switch 5603 b are turned off. At thistime, Dataj+1 input to the wiring 5621_J is input to the signal lineSj+1 through the third switch 5603 c.

As described above, in the signal line driver circuit of FIG. 37, onegate selection period is divided into three; thus, video signals can beinput to three signal lines through one wiring 5621 in one gateselection period. Therefore, in the signal line driver circuit in FIG.37, the number of connections between the substrate provided with thedriver IC 5601 and the substrate provided with the pixel portion can beapproximately one third of the number of signal lines. The number ofconnections is reduced to approximately one third of the number ofsignal lines; therefore, reliability, yield, and the like of the signalline driver circuit in FIG. 37 can be improved.

By applying the signal line driver circuit of this embodiment mode toeach display device described in Embodiment Modes 1 to 4, the number ofconnections between the substrate provided with the pixel portion and anexternal substrate can be further reduced. Therefore, reliability andyield of the display device of this embodiment mode can be improved.

Next, a case where n-channel transistors are used as the first switch5603 a, the second switch 5603 b, and the third switch 5603 c isdescribed with reference to FIG. 39. Note that components similar tothose of FIG. 37 are denoted by the same reference numerals, anddetailed description of the same portions and portions having similarfunctions is omitted.

A first transistor 5903 a corresponds to the first switch 5603 a. Asecond transistor 5903 b corresponds to the second switch 5603 b. Athird transistor 5903 c corresponds to the third switch 5603 c.

For example, in the case of the switch group 5602_J, a first electrodeof the first transistor 5903 a is connected to the wiring 5621_J, asecond electrode of the first transistor 5903 a is connected to thesignal line Sj−1, and a gate electrode of the first transistor 5903 a isconnected to the first wiring 5611. A first electrode of the secondtransistor 5903 b is connected to the wiring 5621_J, a second electrodeof the second transistor 5903 b is connected to the signal line Sj, anda gate electrode of the second transistor 5903 b is connected to thesecond wiring 5612. A first electrode of the third transistor 5903 c isconnected to the wiring 5621_J, a second electrode of the thirdtransistor 5903 c is connected to the signal line Sj+1, and a gateelectrode of the third transistor 5903 c is connected to the thirdwiring 5613.

The first transistor 5903 a, the second transistor 5903 b, and the thirdtransistor 5903 c each function as a switching transistor. Further, eachof the first transistor 5903 a, the second transistor 5903 b, and thethird transistor 5903 c is turned on when a signal input to each gateelectrode is at the H level, and is turned off when a signal input toeach gate electrode is at the L level.

When n-channel transistors are used as the first switch 5603 a, thesecond switch 5603 b, and the third switch 5603 c, amorphous silicon canbe used for a semiconductor layer of each transistor; thus,simplification of a manufacturing process, reduction in manufacturingcost, and improvement in yield can be achieved. Further, a semiconductordevice such as a large-scale display panel can be formed. Even whenpolysilicon or single crystalline silicon is used for the semiconductorlayer of the transistor, simplification of a manufacturing process canalso be realized.

In the signal line driver circuit of FIG. 39, n-channel transistors areused as the first transistor 5903 a, the second transistor 5903 b, andthe third transistor 5903 c; however, p-channel transistors may be usedas the first transistor 5903 a, the second transistor 5903 b, and thethird transistor 5903 c. In that case, each transistor is turned on whena signal input to the gate electrode is at the L level, and is turnedoff when a signal input to the gate electrode is at the H level.

Note that the arrangement, the number, a driving method, and the like ofswitches are not limited as long as one gate selection period is dividedinto a plurality of sub-selection periods and video signals are input toa plurality of signal lines through one wiring in each of the pluralityof sub-selection periods as shown in FIG. 37.

For example, when video signals are input to three or more signal linesthrough one wiring in each of three or more sub-selection periods, aswitch and a wiring for controlling the switch may be additionallyprovided. Note that when one gate selection period is divided into fouror more sub-selection periods, each sub-selection period becomes tooshort. Therefore, one gate selection period is preferably divided intotwo or three sub-selection periods.

As another example, as shown in a timing chart of FIG. 40, one gateselection period may be divided into a precharge period Tp, the firstsub-selection period T1, the second sub-selection period T2, and thethird sub-selection period T3. The timing chart of FIG. 40 shows timingat which the scan line Gi in the i-th row is selected, timing 5803 a atwhich the first switch 5603 a is turned on or off, timing 5803 b atwhich the second switch 5603 b is turned on or off, timing 5803 c atwhich the third switch 5603 c is turned on or off, and a signal 5821_Jinput to the wiring 5621_J in the J-th column. As shown in FIG. 40, thefirst switch 5603 a, the second switch 5603 b, and the third switch 5603c are tuned on in the precharge period Tp. At this time, a prechargevoltage Vp input to the wiring 5621_J is input to each of the signalline Sj−1, the signal line Sj, and the signal line Sj+1 through thefirst switch 5603 a, the second switch 5603 b, and the third switch 5603c. In the first sub-selection period T1, the first switch 5603 a isturned on, and the second switch 5603 b and the third switch 5603 c areturned off. At this time, Dataj−1 input to the wiring 5621_J is input tothe signal line Sj−1 through the first switch 5603 a. In the secondsub-selection period T2, the second switch 5603 b is turned on, and thefirst switch 5603 a and the third switch 5603 c are turned off. At thistime, Dataj input to the wiring 5621_J is input to the signal line Sjthrough the second switch 5603 b. In the third sub-selection period T3,the third switch 5603 c is turned on, and the first switch 5603 a andthe second switch 5603 b are turned off. At this time, Dataj+1 input tothe wiring 5621_J is input to the signal line Sj+1 through the thirdswitch 5603 c.

As described above, in the signal line driver circuit of FIG. 37, towhich the timing chart of FIG. 40 is applied, a signal line can beprecharged by providing a precharge selection period beforesub-selection periods. Thus, a video signal can be written to a pixelwith high speed. Note that components similar to those in FIG. 38 aredenoted by the same reference numerals, and detailed description of thesame portions and portions having similar functions is omitted.

Also in FIG. 41, one gate selection period can be divided into aplurality of sub-selection periods and video signals can be input to aplurality of signal lines through one wiring in each of the plurality ofsub-selection periods as shown in FIG. 37. Note that FIG. 41 shows onlya switch group 6022_J in the J-th column in a signal line drivercircuit. The switch group 6022_J includes a first transistor 6001, asecond transistor 6002, a third transistor 6003, a fourth transistor6004, a fifth transistor 6005, and a sixth transistor 6006. The firsttransistor 6001, the second transistor 6002, the third transistor 6003,the fourth transistor 6004, the fifth transistor 6005, and the sixthtransistor 6006 are n-channel transistors. The switch group 6022_J isconnected to a first wiring 6011, a second wiring 6012, a third wiring6013, a fourth wiring 6014, a fifth wiring 6015, a sixth wiring 6016,the wiring 5621_J, the signal line Sj−1, the signal line Sj, and thesignal line Sj+1.

A first electrode of the first transistor 6001 is connected to thewiring 5621_J, a second electrode of the first transistor 6001 isconnected to the signal line Sj−1, and a gate electrode of the firsttransistor 6001 is connected to the first wiring 6011. A first electrodeof the second transistor 6002 is connected to the wiring 5621_J, asecond electrode of the second transistor 6002 is connected to thesignal line Sj−1, and a gate electrode of the second transistor 6002 isconnected to the second wiring 6012. A first electrode of the thirdtransistor 6003 is connected to the wiring 5621_J, a second electrode ofthe third transistor 6003 is connected to the signal line Sj, and a gateelectrode of the third transistor 6003 is connected to the third wiring6013. A first electrode of the fourth transistor 6004 is connected tothe wiring 5621_J, a second electrode of the fourth transistor 6004 isconnected to the signal line Sj, and a gate electrode of the fourthtransistor 6004 is connected to the fourth wiring 6014. A firstelectrode of the fifth transistor 6005 is connected to the wiring5621_J, a second electrode of the fifth transistor 6005 is connected tothe signal line Sj+1, and a gate electrode of the fifth transistor 6005is connected to the fifth wiring 6015. A first electrode of the sixthtransistor 6006 is connected to the wiring 5621_J, a second electrode ofthe sixth transistor 6006 is connected to the signal line Sj+1, and agate electrode of the sixth transistor 6006 is connected to the sixthwiring 6016.

The first transistor 6001, the second transistor 6002, the thirdtransistor 6003, the fourth transistor 6004, the fifth transistor 6005,and the sixth transistor 6006 each function as a switching transistor.Further, each of first transistor 6001, the second transistor 6002, thethird transistor 6003, the fourth transistor 6004, the fifth transistor6005, and the sixth transistor 6006 is turned on when a signal input toeach gate electrode is at the H level, and is turned off when a signalinput to each gate electrode is at the L level.

The first wiring 6011 and the second wiring 6012 correspond to the firstwiring 5611 in FIG. 39. The third wiring 6013 and the fourth wiring 6014correspond to the second wiring 5612 in FIG. 39. The fifth wiring 6015and the sixth wiring 6016 correspond to the third wiring 5613 in FIG.39. Note that the first transistor 6001 and the second transistor 6002correspond to the first transistor 5903 a in FIG. 39. The thirdtransistor 6003 and the fourth transistor 6004 correspond to the secondtransistor 5903 b in FIG. 39. The fifth transistor 6005 and the sixthtransistor 6006 correspond to the third transistor 5903 c in FIG. 39.

In FIG. 41, in the first sub-selection period T1 shown in FIG. 38, oneof the first transistor 6001 and the second transistor 6002 is turnedon. In the second sub-selection period T2, one of the third transistor6003 and the fourth transistor 6004 is turned on. In the thirdsub-selection period T3, one of the fifth transistor 6005 and the sixthtransistor 6006 is turned on. Further, in the precharge period Tp shownin FIG. 40, either the first transistor 6001, the third transistor 6003,and the fifth transistor 6005 or the second transistor 6002, the fourthtransistor 6004, and the sixth transistor 6006 are turned on.

Thus, in FIG. 41, since the on time of each transistor can be shortened,deterioration in characteristics of the transistor can be suppressed.This is because in the first sub-selection period T1 shown in FIG. 38,for example, the video signal can be input to the signal line Sj−1 whenone of the first transistor 6001 and the second transistor 6002 isturned on. In the first sub-selection period T1 shown in FIG. 38, forexample, when both the first transistor 6001 and the second transistor6002 are turned on at the same time, the video signal can be input tothe signal line Sj−1 with high speed.

FIG. 41 illustrates the case where two transistors are connected inparallel between the wiring 5621 and the signal line. However, theinvention is not limited thereto, and three or more transistors may beconnected in parallel between the wiring 5621 and the signal line.Accordingly, deterioration in characteristics of each transistor can befurther suppressed.

Although this embodiment mode has been described with reference tovarious drawings, the contents (or part of the contents) described ineach drawing can be freely applied to, combined with, or replaced withthe contents (or part of the contents) described in another drawing.Further, much more drawings can be formed by combining each part in theabove-described drawings with another part.

Similarly, the contents (or part of the contents) described in eachdrawing in this embodiment mode can be freely applied to, combined with,or replaced with the contents (or part of the contents) described in adrawing in another embodiment mode. Further, much more drawings can beformed by combining each part in the drawings in this embodiment modewith part of another embodiment mode.

Note that this embodiment mode has described just examples of embodying,slightly transforming, modifying, improving, describing in detail, orapplying the contents (or part of the contents) described in otherembodiment modes, an example of related part thereof, or the like.Therefore, the contents described in other embodiment modes can befreely applied to, combined with, or replaced with the contentsdescribed in this embodiment mode.

Embodiment Mode 6

This embodiment mode describes a structure for preventing a defect dueto electrostatic discharge damage in each display device described inEmbodiment Modes 1 to 4.

Note that electrostatic discharge damage refers to damage caused by alarge current flow within a semiconductor device due to instantdischarge of positive or negative charges stored in a human body or anobject through an input/output terminal of the semiconductor device whenin contact with the semiconductor device.

FIG. 42A shows a structure for preventing electrostatic discharge damagecaused in a scan line by a protective diode. FIG. 42A shows a structurewhere the protective diode is provided between a wiring 6111 and thescan line. Although not shown, a plurality of pixels are connected tothe scan line Gi in the i-th row. A transistor 6101 is used as theprotective diode. The transistor 6101 is an n-channel transistor;however, a p-channel transistor may be used, and the polarity of thetransistor 6101 may be the same as that of a transistor included in ascan line driver circuit or a pixel.

A single protective diode is provided here; however, a plurality ofprotective diodes may be arranged in series, in parallel, or inseries-parallel.

A first electrode of the transistor 6101 is connected to the scan lineGi in the i-th row, a second electrode of the transistor 6101 isconnected to the wiring 6111, and a gate electrode of the transistor6101 is connected to the scan line Gi in the i-th row.

An operation of the structure in FIG. 42A is described. A certainpotential is input to the wiring 6111, which is lower than the L levelof a signal input to the scan line Gi in the i-th row. When positive ornegative charges are not discharged to the scan line Gi in the i-th row,a potential of the scan line Gi in the i-th row is at the H level or theL level, so that the transistor 6101 is turned off. On the other hand,when negative charges are discharged to the scan line Gi in the i-throw, the potential of the scan line Gi in the i-th row decreasesinstantaneously. At this time, if the potential of the scan line Gi inthe i-th row becomes lower than a value obtained by subtracting athreshold voltage of the transistor 6101 from a potential of the wiring6111, the transistor 6101 is turned on, and thus a current flows to thewiring 6111 through the transistor 6101. Therefore, the structure shownin FIG. 42A can prevent a large current from flowing to the pixel, sothat electrostatic discharge damage of the pixel can be prevented.

FIG. 42B shows a structure for preventing electrostatic discharge damagewhen positive charges are discharged to the scan line Gi in the i-throw. A transistor 6102 functioning as a protective diode is providedbetween a scan line and a wiring 6112. Note that a single protectivediode is provided here; however, a plurality of protective diodes may bearranged in series, in parallel, or in series-parallel. The transistor6102 is an n-channel transistor; however, a p-channel transistor may beused, and the polarity of the transistor 6102 may be the same as that ofthe transistor included in the scan line driver circuit or the pixel. Afirst electrode of the transistor 6102 is connected to the scan line Giin the i-th row, a second electrode of the transistor 6102 is connectedto the wiring 6112, and a gate electrode of the transistor 6102 isconnected to the wiring 6112. Note that a potential higher than the Hlevel of the signal input to the scan line Gi in the i-th row is inputto the wiring 6112. Therefore, when charges are not discharged to thescan line Gi in the i-th row, the transistor 6102 is turned off. On theother hand, when positive charges are discharged to the scan line Gi inthe i-th row, the potential of the scan line Gi in the i-th rowincreases instantaneously. At this time, if the potential of the scanline Gi in the i-th row becomes higher than the sum of a potential ofthe wiring 6112 and a threshold voltage of the transistor 6102, thetransistor 6102 is turned on, and thus a current flows to the wiring6112 through the transistor 6102. Therefore, the structure shown in FIG.42B can prevent a large current from flowing to the pixel, so thatelectrostatic discharge damage of the pixel can be prevented.

As shown in FIG. 42C, with a structure which combines the structures inFIGS. 42A and 42B, electrostatic discharge damage of the pixel can beprevented even when positive or negative charges are discharged to thescan line Gi in the i-th row. Note that components similar to those inFIGS. 42A and 42B are denoted by common reference numerals, and detaileddescription of the same portions and portions having similar functionsis omitted.

FIG. 43A shows a structure where a transistor 6201 functioning as aprotective diode is connected between a scan line and a storagecapacitor line. Note that a single protective diode is provided here;however, a plurality of protective diodes may be arranged in series, inparallel, or in series-parallel. The transistor 6201 is an n-channeltransistor; however, a p-channel transistor may be used. The polarity ofthe transistor 6201 may be the same as that of the transistor includedin the scan line driver circuit or the pixel. Note that a wiring 6211functions as a storage capacitor line. A first electrode of thetransistor 6201 is connected to the scan line Gi in the i-th row, asecond electrode of the transistor 6201 is connected to the wiring 6211,and a gate electrode of the transistor 6201 is connected to the scanline Gi in the i-th row. Note that a potential lower than the L level ofthe signal input to the scan line Gi in the i-th row is input to thewiring 6211. Therefore, when charges are not discharged to the scan lineGi in the i-th row, the transistor 6210 is turned off. On the otherhand, when negative charges are discharged to the scan line Gi in thei-th row, the potential of the scan line Gi in the i-th row decreasesinstantaneously. At this time, if the potential of the scan line Gi inthe i-th row becomes lower than a value obtained by subtracting athreshold voltage of the transistor 6201 from a potential of the wiring6211, the transistor 6201 is turned on, and thus a current flows to thewiring 6211 through the transistor 6201. Therefore, the structure shownin FIG. 43A can prevent a large current from flowing to the pixel, sothat electrostatic discharge damage of the pixel can be prevented.Further, since the storage capacitor line is utilized for dischargingcharges in the structure shown in FIG. 43A, an additional wiring is notrequired to be provided.

FIG. 43B shows a structure for preventing electrostatic discharge damagewhen positive charges are discharged to the scan line Gi in the i-throw. Here, a potential higher than the H level of the signal input tothe scan line Gi in the i-th row is input to the wiring 6211. Therefore,when charges are not discharged to the scan line Gi in the i-th row, atransistor 6202 is turned off. On the other hand, when positive chargesare discharged to the scan line Gi in the i-th row, the potential of thescan line Gi in the i-th row increases instantaneously. At this time, ifthe potential of the scan line Gi in the i-th row becomes higher thanthe sum of a potential of the wiring 6211 and a threshold voltage of thetransistor 6202, the transistor 6202 is turned on, and thus a currentflows to the wiring 6211 through the transistor 6202. Therefore, thestructure shown in FIG. 43B can prevent a large current from flowing tothe pixel, so that electrostatic discharge damage of the pixel can beprevented. Further, because the storage capacitor line is utilized fordischarging charges in the structure shown in FIG. 43B, an additionalwiring is not needed to be provided. Note that components similar tothose in FIG. 43A are denoted by common reference numerals, and detaileddescription of the same portions and portions having similar functionsis omitted.

Next, FIG. 44A shows a structure for preventing electrostatic dischargedamage caused in a signal line by a protective diode. FIG. 44A shows astructure where the protective diode is provided between a wiring 6411and the signal line. Although not shown, a plurality of pixels areconnected to the signal line Sj in the j-th column. A transistor 6401 isused as the protective diode. The transistor 6401 is an n-channeltransistor; however, a p-channel transistor may be used. The polarity ofthe transistor 6401 may be the same as that of a transistor included ina signal line driver circuit or the pixel.

Note that a single protective diode is provided here; however, aplurality of protective diodes may be arranged in series, in parallel,or in series-parallel.

A first electrode of the transistor 6401 is connected to the signal lineSj in the j-th column, a second electrode of the transistor 6401 isconnected to the wiring 6411, and a gate electrode of the transistor6401 is connected to the signal line Sj in the j-th column.

An operation of the structure in FIG. 44A is described. A certainpotential is input to the wiring 6411, which is lower than the leastvalue of a video signal input to the signal line Sj in the j-th column.When positive or negative charges are not discharged to the signal lineSj in the j-th column, a potential of the signal line Sj in the j-thcolumn is the same as that of the video signal, so that the transistor6401 is turned off. On the other hand, when negative charges aredischarged to the signal line Sj in the j-th column, the potential ofthe signal line Sj in the j-th column decreases instantaneously. At thistime, if the potential of the signal line Sj in the j-th column becomeslower than a value obtained by subtracting a threshold voltage of thetransistor 6401 from a potential of the wiring 6411, the transistor 6401is turned on, and thus a current flows to the wiring 6411 through thetransistor 6401. Therefore, the structure shown in FIG. 44A can preventa large current from flowing to the pixel, so that electrostaticdischarge damage of the pixel can be prevented.

FIG. 44B shows a structure for preventing electrostatic discharge damagewhen positive charges are discharged to the signal line Sj in the j-thcolumn. A transistor 6402 functioning as a protective diode is providedbetween the signal line and a wiring 6412. Note that a single protectivediode is provided here; however, a plurality of protective diodes may bearranged in series, in parallel, or in series-parallel. The transistor6402 is an n-channel transistor; however, a p-channel transistor may beused. The polarity of the transistor 6402 may be the same as that of thetransistor included in the signal line driver circuit or the pixel. Afirst electrode of the transistor 6402 is connected to the signal lineSj in the j-th column, a second electrode of the transistor 6402 isconnected to the wiring 6412, and a gate electrode of the transistor6402 is connected to the wiring 6412. Note that a potential higher thanthe greatest value of a video signal input to the signal line Sj in thej-th column is input to the wiring 6412. Therefore, when charges are notdischarged to the signal line Sj in the j-th column, the transistor 6402is turned off. On the other hand, when positive charges are dischargedto the signal line Sj in the j-th column, the potential of the signalline Sj in the j-th column increases instantaneously. At this time, ifthe potential of the signal line Sj in the j-th column is higher thanthe sum of a potential of the wiring 6412 and a threshold voltage of thetransistor 6402, the transistor 6402 is turned on, and thus a currentflows to the wiring 6412 through the transistor 6402. Therefore, thestructure shown in FIG. 44B can prevent a large current from flowing tothe pixel, so that electrostatic discharge damage of the pixel can beprevented.

As shown in FIG. 44C, with a structure which combines the structures inFIGS. 44A and 44B, electrostatic discharge damage of the pixel can beprevented even when either positive or negative charges are dischargedto the signal line Sj in the j-th column. Note that components similarto those in FIGS. 44A and 44B are denoted by common reference numerals,and detailed description of the same portions and portions havingsimilar functions is omitted.

This embodiment mode describes the structures for preventingelectrostatic discharge damage of the pixel connected to the scan lineand the signal line. However, the structure in this embodiment mode isnot only used for preventing electrostatic discharge damage of the pixelconnected to the scan line and the signal line. For example, when thisembodiment mode is used for the wiring to which a signal or a potentialis input, which is connected to the scan line driver circuit and thesignal line driver circuit described in Embodiment Modes 1 to 4,electrostatic discharge damage of the scan line driver circuit and thesignal line driver circuit can be prevented.

Although this embodiment mode has been described with reference tovarious drawings, the contents (or part of the contents) described ineach drawing can be freely applied to, combined with, or replaced withthe contents (or part of the contents) described in another drawing.Further, much more drawings can be formed by combining each part in theabove-described drawings with another part.

Similarly, the contents (or part of the contents) described in eachdrawing in this embodiment mode can be freely applied to, combined with,or replaced with the contents (or part of the contents) described in adrawing in another embodiment mode. Further, much more drawings can beformed by combining each part in the drawings in this embodiment modewith part of another embodiment mode.

Note that this embodiment mode has described just examples of embodying,slightly transforming, modifying, improving, describing in detail, orapplying the contents (or part of the contents) described in otherembodiment modes, an example of related part thereof, or the like.Therefore, the contents described in other embodiment modes can befreely applied to, combined with, or replaced with the contentsdescribed in this embodiment mode.

Embodiment Mode 7

This embodiment mode describes another structure of a display devicewhich can be applied to each display device described in EmbodimentModes 1 to 4.

FIG. 45A shows a structure where a diode-connected transistor isprovided between a scan line and another scan line. FIG. 45A shows astructure where a diode-connected transistor 6301 a is provided betweenthe scan line Gi−1 in the (i−1)th row and the scan line Gi in the i-throw, and a diode-connected transistor 6301 b is provided between thescan line Gi in the i-th row and the scan line Gi+1 in the (i+1)th row.Note that the transistors 6301 a and 6301 b are n-channel transistors;however, p-channel transistors may be used. The polarity of thetransistors 6301 a and 6301 b may be the same as that of a transistorincluded in a scan line driver circuit or a pixel.

Note that FIG. 45A typically shows the scan line Gi−1 in the (i−1)throw, the scan line Gi in the i-th row, and the scan line Gi+1 in the(i+1)th row, but a diode-connected transistor is similarly providedbetween other scan lines.

A first electrode of the transistor 6301 a is connected to the scan lineGi in the i-th row, a second electrode of the transistor 6301 a isconnected to the scan line Gi−1 in the (i−1)th row, and a gate electrodeof the transistor 6301 a is connected to the scan line Gi−1 in the(i−1)th row. A first electrode of the transistor 6301 b is connected tothe scan line Gi+1 in the (i+1)th row, a second electrode of thetransistor 6301 b is connected to the scan line Gi in the i-th row, anda gate electrode of the transistor 6301 b is connected to the scan lineGi in the i-th row.

An operation of the structure in FIG. 45A is described. In each scanline driver circuit described in Embodiment Modes 1 to 4, the scan lineGi−1 in the (i−1)th row, the scan line Gi in the i-th row, and the scanline Gi+1 in the (i+1)th row are maintained at the L level in thenon-selection period. Therefore, the transistors 6301 a and 6301 b areturned off. However, when the potential of the scan line Gi in the i-throw is increased due to noise or the like, a pixel is selected by thescan line Gi in the i-th row and a wrong video signal is written to thepixel. By providing the diode-connected transistor between the scanlines as shown in FIG. 45A, writing of a wrong video signal to the pixelcan be prevented. This is because when the potential of the scan line Giin the i-th row is increased to be equal to or higher than the sum of apotential of the scan line Gi−1 in the (i−1)th row and a thresholdvoltage of the transistor 6301 a, the transistor 6301 a is turned on andthe potential of the scan line Gi in the i-th row is decreased; thus, apixel is not selected by the scan line Gi in the i-th row.

The structure of FIG. 45A is particularly advantageous when a scan linedriver circuit and a pixel portion are formed over the same substrate,because in the scan line driver circuit including only n-channeltransistors or only p-channel transistors, a scan line is sometimes in afloating state and noise is easily generated in the scan line.

FIG. 45B shows a structure where the direction of the diode-connectedtransistors provided between the scan lines is reversed with respect tothat in FIG. 45A. Note that transistors 6302 a and 6302 b are n-channeltransistors; however, p-channel transistors may be used. The polarity ofthe transistors 6302 a and 6302 b may be the same as that of thetransistor included in the scan line driver circuit or the pixel. InFIG. 45B, a first electrode of the transistor 6302 a is connected to thescan line Gi in the i-th row, a second electrode of the transistor 6302a is connected to the scan line Gi−1 in the (i−1)th row, and a gateelectrode of the transistor 6302 a is connected to the scan line Gi inthe i-th row. A first electrode of the transistor 6302 b is connected tothe scan line Gi+1 in the (i+1)th row, a second electrode of thetransistor 6302 b is connected to the scan line Gi in the i-th row, anda gate electrode of the transistor 6302 b is connected to the scan lineGi+1 in the (i+1)th row. In FIG. 45B, similarly to FIG. 44A, when thepotential of the scan line Gi in the i-th row is increased to be equalto or higher than the sum of the potential of the scan line Gi+1 in the(i+1)th row and a threshold voltage of the transistor 6302 b, thetransistor 6302 b is turned on and the potential of the scan line Gi inthe i-th row is decreased. Thus, a pixel is not selected by the scanline Gi in the i-th row, and writing of a wrong video signal to thepixel can be prevented.

As shown in FIG. 45C, with a structure which combines the structures inFIGS. 45A and 45B, even when the potential of the scan line Gi in thei-th row is increased, the transistors 6301 a and 6301 b are tuned on,so that the potential of the scan line Gi in the i-th row is decreased.Note that in FIG. 45C, since a current flows through two transistors,larger noise can be removed. Note that components similar to those inFIGS. 45A and 45B are denoted by common reference numerals, and detaileddescription of the same portions and portions having similar functionsis omitted.

Note that when a diode-connected transistor is provided between the scanline and the storage capacitor line as shown in FIGS. 43A and 43B,advantageous effects similar to FIGS. 45A, 45B, and 45C can be obtained.

Although this embodiment mode has been described with reference tovarious drawings, the contents (or part of the contents) described ineach drawing can be freely applied to, combined with, or replaced withthe contents (or part of the contents) described in another drawing.Further, much more drawings can be formed by combining each part in theabove-described drawings with another part.

Similarly, the contents (or part of the contents) described in eachdrawing in this embodiment mode can be freely applied to, combined with,or replaced with the contents (or part of the contents) described in adrawing in another embodiment mode. Further, much more drawings can beformed by combining each part in the drawings in this embodiment modewith part of another embodiment mode.

Note that this embodiment mode has described just examples of embodying,slightly transforming, modifying, improving, describing in detail, orapplying the contents (or part of the contents) described in otherembodiment modes, an example of related part thereof, or the like.Therefore, the contents described in other embodiment modes can befreely applied to, combined with, or replaced with the contentsdescribed in this embodiment mode.

Embodiment Mode 8

Embodiment Mode 8 will describe a structure of a transistor and a methodfor manufacturing the transistor.

FIGS. 46A to 46G illustrate a structure of a transistor and a method formanufacturing the transistor. FIG. 46A illustrates a structural exampleof the transistor. FIGS. 46B to 46G exemplify the manufacturing methodof the transistor.

The structure and the manufacturing method of a transistor are notlimited to those illustrated in FIGS. 46A to 46G, and various structuresand manufacturing methods can be employed.

A structural example of a transistor is described with reference to FIG.46A. FIG. 46A is a cross-sectional view of plural transistors havingdifferent structures. In FIG. 46A, the plural transistors havingdifferent structures are arranged to be apposed; however, thisarrangement is made for describing the structures of the transistors,and it is unnecessary to appose the transistors actually as shown inFIG. 46A, and the transistors can be disposed as necessary.

Then, layers constituting a transistor are each described.

A substrate 110111 can be a glass substrate such as a bariumborosilicate glass or an alumino borosilicate glass, a quartz substrate,a ceramic substrate or a metal substrate including stainless steel, forexample. Besides these, a substrate formed of a synthetic resin havingflexibility such as acrylic or plastic represented by polyethyleneterephthalate (PET), polyethylene naphthalate (PEN), andpolyethersulfone (PES) can be also used. By using such a flexiblesubstrate, a bendable semiconductor device can be manufactured. Aflexible substrate has no significant restrictions on an area and ashape of a substrate to be used, and thus, as the substrate 110111, forexample, a rectangular substrate with a side of one meter or more isused, the productivity can be significantly improved. This merit isgreatly advantageous as compared to the case of using a circular siliconsubstrate.

An insulating film 110112 serves as a base film. The insulating film110112 is provided to prevent alkali metal such as Na or alkaline earthmetal from the substrate 110111 from adversely affecting characteristicsof a semiconductor element. The insulating film 110112 can have asingle-layer structure or a stacked-layer structure of an insulatingfilm(s) containing oxygen or nitrogen, such as silicon oxide (SiO_(x)),silicon nitride (SiN_(x)), silicon oxynitride (SiO_(x)N_(y), x>y), orsilicon nitride oxide (SiN_(x)O_(y), x>y). For example, when theinsulating film 110112 is provided to have a two-layer structure, it ispreferable that a silicon nitride oxide film be used as a firstinsulating film and a silicon oxynitride film be used as a secondinsulating film. When the insulating film 110112 is provided to have athree-layer structure, it is preferable that a silicon oxynitride filmbe used as a first insulating film, a silicon nitride oxide film be usedas a second insulating film, and a silicon oxynitride film be used as athird insulating film.

Semiconductor layers 110113, 110114, and 110115 can be formed using anamorphous semiconductor or a semi-amorphous semiconductor (SAS).Alternatively, a polycrystalline semiconductor layer may be used. SAS isa semiconductor having an intermediate structure between amorphous andcrystalline (including single crystal and polycrystalline) structuresand having a third state which is stable in free energy. Moreover, SASincludes a crystalline region with a short range order and latticedistortion. A crystalline region of 0.5 nm to 20 nm can be observed inat least part of a SAS film. When silicon is contained as a maincomponent, Raman spectrum shifts to a wave number side lower than 520cm⁻³. The diffraction peaks of (111) and (220) which are thought to bederived from a silicon crystalline lattice are observed by X-raydiffraction. SAS contains hydrogen or halogen of at least 1 atomic % ormore to terminate dangling bonds. SAS is formed by glow dischargedecomposition (plasma CVD) of a material gas. When silicon is containedas a main component, as the material gas, Si₂H₆, SiH₂Cl₂, SiHCl₃, SiCl₄,SiF₄, or the like can be used in addition to SiH₄. Further, GeF₄ may bemixed. Alternatively, the material gas may be diluted with H₂, or H₂ andone or more kinds of rare gas elements selected from He, Ar, Kr, and Ne.A dilution ratio may be in the range of 2 to 1000 times, pressure may bein the range of approximately 0.1 to 133 Pa, a power supply frequencymay be 1 MHz to 120 MHz, preferably 13 MHz to 60 MHz, and a substrateheating temperature may be 300° C. or lower. A concentration ofimpurities in atmospheric components such as oxygen, nitrogen, andcarbon is preferably 1×10²⁰ cm⁻³ or less as impurity elements in thefilm. In particular, an oxygen concentration is 5×10¹⁹/cm³ or less,preferably 1×10¹⁹/cm³ or less. Here, an amorphous semiconductor film isformed using a material containing silicon (Si) as its main component(e.g., Si_(x)Ge_(1-x)) by a sputtering method, an LPCVD method, a plasmaCVD method, or the like. Then, the amorphous semiconductor film iscrystallized by a crystallization method such as a laser crystallizationmethod, a thermal crystallization method using RTA or an annealingfurnace, or a thermal crystallization method using a metal element whichpromotes crystallization.

An insulating film 110116 can have a single-layer structure or astacked-layer structure of an insulating film(s) containing oxygen ornitrogen, such as silicon oxide (SiO_(x)), silicon nitride (SiN_(x)),silicon oxynitride (SiO_(x)N_(y), x>y), or silicon nitride oxide(SiN_(x)O_(y), x>y).

A gate electrode 110117 can have a single-layer structure of aconductive film or a stacked-layer structure of two or three conductivefilms. As a material for the gate electrode 110117, a conductive filmcan be used. For example, a film of an element such as tantalum (Ta),titanium (Ti), molybdenum (Mo), tungsten (W), chromium (Cr), or silicon(Si); a nitride film containing the element (typically, a tantalumnitride film, a tungsten nitride film, or a titanium nitride film); analloy film of a combination of the elements (typically, a Mo—W alloy ora Mo—Ta alloy); a silicide film containing the element (typically, atungsten silicide film or a titanium silicide film); and the like can beused. Note that the aforementioned film, nitride film, alloy film,silicide film, or the like can have a single-layer structure or astacked-layer structure.

An insulating film 110118 can have a single-layer structure or astacked-layer structure of an insulating film containing oxygen ornitrogen, such as silicon oxide (SiO_(x)), silicon nitride (SiN_(x)),silicon oxynitride (SiO_(x)N_(y), x>y), or silicon nitride oxideSiN_(x)O_(y), x>y); or a film containing carbon, such as DLC(Diamond-Like Carbon), by a sputtering method, a plasma CVD method, orthe like.

An insulating film 110119 can have a single-layer structure or astacked-layer structure of a siloxane resin; an insulating filmcontaining oxygen or nitrogen, such as silicon oxide (SiO_(x)), siliconnitride (SiN_(x)), silicon oxynitride (SiO_(x)N_(y), x>y), or siliconnitride oxide (SiN_(x)O_(y), x>y); a film containing carbon, such as DLC(Diamond-Like Carbon); or an organic material such as epoxy, polyimide,polyamide, polyvinyl phenol, benzocyclobutene, or acrylic. Note that thesiloxane resin corresponds to a resin having Si—O—Si bonds. Siloxaneincludes a skeleton structure of a bond of silicon (Si) and oxygen (O).As a substituent, an organic group containing at least hydrogen (such asan alkyl group or an aryl group) is used. Alternatively, a fluoro group,or a fluoro group and an organic group containing at least hydrogen canbe used as a substituent. Note that the insulating film 110119 can beprovided to cover the gate electrode 110117 directly without provisionof the insulating film 110118.

As a conductive film 110123, a film of an element such as Al, Ni, C, W,Mo, Ti, Pt, Cu, Ta, Au, or Mn, a nitride film containing the element, analloy film of a combination of the elements, a silicide film containingthe element, or the like can be used. For example, as an alloycontaining some of such elements, an Al alloy containing C and Ti, an Alalloy containing Ni, an Al alloy containing C and Ni, an Al alloycontaining C and Mn, or the like can be used. In the case of astacked-layer structure, for example, a structure can be such that Al isinterposed between Mo, Ti, or the like; thus, resistance of Al to heator chemical reaction can be improved.

Next, characteristics of each structure is described with reference tothe cross-sectional view of the plurality of transistors each having adifferent structure in FIG. 46A.

Reference numeral 110101 denotes a single drain transistor. Since it canbe formed by a simple method, it is advantageous in low manufacturingcost and high yield. Here, the semiconductor layers 110113 and 110115each have different concentrations of impurities, and the semiconductorlayer 110113 is used as a channel region and the semiconductor layer110115 is used as source region and a drain region. By controlling theamount of impurities in this manner, resistivity of the semiconductorlayer can be controlled. An electrical connection state between thesemiconductor layer and the conductive film 110123 can be closer toohmic contact. Note that as a method of separately forming thesemiconductor layers each including different amount of impurities, amethod where impurities are added to the semiconductor layers using thegate electrode 110117 as a mask can be used.

Reference numeral 110102 denotes a transistor in which the gateelectrode 110117 has a taper angle of certain degrees or more (which isequal to or larger than 45° to smaller than 95°, more preferably, equalto or larger than 60° to smaller than 95°, or may be smaller than 45°).Since it can be formed by a simple method, it is advantageous in lowmanufacturing cost and high yield. Here, the semiconductor layers110113, 110114, and 110115 each have different concentration ofimpurities, and the semiconductor layers 110113, 110114, and 110115 areused as a channel region, a lightly doped drain (LDD) region, and asource region and a drain region, respectively. By controlling theamount of impurities in this manner, resistivity of the semiconductorlayer can be controlled. An electrical connection state between thesemiconductor layer and the conductive film 110123 can be closer toohmic contact. Since the transistor includes the LDD region, highelectric field is hardly applied in the transistor, so thatdeterioration of the element due to hot carriers can be suppressed. Notethat as a method of separately forming the semiconductor layers eachhaving different amount of impurities, a method where impurities areadded to the semiconductor layers using the gate electrode 110117 as amask can be used. In the transistor 110102, since the gate electrode110117 has a taper angle of certain degrees or more, gradient of theconcentration of impurities added to the semiconductor layer through thegate electrode 110117 can be provided, and the LDD region can be easilyformed.

Reference numeral 110103 denotes a transistor in which the gateelectrode 110117 includes at least two layers and a lower gate electrodeis longer than an upper gate electrode. In this specification, the shapeof the upper gate electrode and the lower gate electrode is referred toas a hat shape. When the gate electrode 110117 has such a hat shape, anLDD region can be formed without addition of a photomask. Note that astructure where the LDD region overlaps with the gate electrode 110117,like the transistor 110103, is particularly called a GOLD (GateOverlapped LDD) structure. As a method of forming the gate electrode110117 with such a hat shape, the following method may be used.

First, when the gate electrode 110117 is patterned, the lower and uppergate electrodes are etched by dry etching so that side surfaces thereofare inclined (tapered). Then, the upper gate electrode is processed byanisotropic etching so that the inclination thereof becomes almostperpendicular. Thus, the gate electrode is formed such that the crosssection is hat-shaped. Then, doping of impurity elements is conductedtwice, so that the semiconductor layer 110113 used as a channel region,the semiconductor layers 110114 used as LDD regions, and thesemiconductor layers 110115 used as a source region and a drain regionare formed.

Note that a portion of the LDD region, which overlaps with the gateelectrode 110117, is referred to as an Lov region, and a portion of theLDD region, which does not overlap with the gate electrode 110117, isreferred to as an Loff region. The Loff region is highly effective insuppressing an off-current value, whereas it is not very effective inpreventing deterioration in an on-current value due to hot carriers byrelieving an electric field in the vicinity of the drain. On the otherhand, the Lov region is highly effective in preventing deterioration inthe on-current value by relieving the electric field in the vicinity ofthe drain, whereas it is not very effective in suppressing theoff-current value. Thus, it is preferable to form a transistor having asuitable structure for characteristics required for each of the variouscircuits. For example, when the semiconductor device is used for adisplay device, a transistor having an Loff region is preferably used asa pixel transistor in order to suppress the off-current value. On theother hand, as a transistor in a peripheral circuit, a transistor havingan Lov region is preferably used in order to prevent deterioration inthe on-current value by relieving the electric field in the vicinity ofthe drain.

Reference numeral 110104 denotes a transistor including a sidewall110121 in contact with a side surface of the gate electrode 110117. Whenthe transistor includes the sidewall 110121, a region overlapping withthe sidewall 110121 can be formed as an LDD region.

Reference numeral 110105 denotes a transistor in which an LDD (Loff)region is formed by doping the semiconductor layer with use of a mask.Thus, the LDD region can surely be formed, and an off-current value ofthe transistor can be reduced.

Reference numeral 110106 denotes a transistor in which an LDD (Lov)region is formed by doping the semiconductor layer with use of a mask.Thus, the LDD region can surely be formed, and deterioration in anon-current value can be prevented by relieving the electric field in thevicinity of the drain of the transistor.

Next, an example of a manufacturing method of a transistor is describedwith reference to FIGS. 46B to 46G.

Note that a structure and a manufacturing method of a transistor are notlimited to those in FIGS. 46A to 46G, and various structures andmanufacturing methods can be used.

In this embodiment mode, a surface of the substrate 110111, theinsulating film 110112, the semiconductor layer 110113, thesemiconductor layer 110114, the semiconductor layer 110115, theinsulating film 110116, the insulating film 110118, or the insulatingfilm 110119 is oxidized or nitrided by plasma treatment, so that thesemiconductor layer or the insulating film can be oxidized or nitrided.By oxidizing or nitriding the semiconductor layer or the insulating filmby plasma treatment in such a manner, a surface of the semiconductorlayer or the insulating film cab be modified, and the insulating filmcan be formed to be denser than an insulating film formed by a CVDmethod or a sputtering method; thus, a defect such as a pinhole can besuppressed, and characteristics and the like of the semiconductor devicecan be improved.

Note that silicon oxide (SiO_(x)) or silicon nitride (SiN_(x)) can beused for the sidewall 110121. As a method of forming the sidewall 110121on the side surface of the gate electrode 110117, a method in which thegate electrode 110117 is formed, then, a silicon oxide (SiO_(x)) film ora silicon nitride (SiN_(x)) film is formed, and then, the silicon oxide(SiO_(x)) film or the silicon nitride (SiN_(x)) film is etched byanisotropic etching can be used, for example. Thus, the silicon oxide(SiO_(x)) film or the silicon nitride (SiN_(x)) film remains only on theside surface of the gate electrode 110117, so that the sidewall 110121can be formed on the side surface of the gate electrode 110117.

FIG. 50 illustrates cross-sectional structures of a bottom gatetransistor and a capacitor.

A first insulating film (an insulating film 110502) is formed entirelyover a substrate 110501. However, the present invention is not limitedto this. The first insulating film (the insulating film 110502) is notnecessarily formed in some cases. The first insulating film can preventimpurities from the substrate from adversely affecting a semiconductorlayer and changing a property of a transistor. In other words, the firstinsulating film serves as a base film. Therefore, a highly reliabletransistor can be manufactured. As the first insulating film, a singlelayer or a stacked layer of a silicon oxide film, a silicon nitridefilm, and/or a silicon oxynitride (SiO_(x)N_(y)) film can be used.

A first conductive layer (a conductive layer 110503 and a conductivelayer 110504) is formed over the first insulating film. The conductivelayer 110503 includes a portion which acts as a gate electrode of atransistor 110520. The conductive layer 110504 includes a portion whichacts as a first electrode of a capacitor 110521. As the first conductivelayer, Ti, Mo, Ta, Cr, W, Al, Nd, Cu, Ag, Au, Pt, Nb, Si, Zn, Fe, Ba,Ge, or an alloy of these elements can be used. Alternatively, a stackedlayer including any of these (including an alloy thereof) can be used.

A second insulating film (an insulating film 110514) is formed to coverat least the first conductive layer. The second insulating film servesas a gate insulating film. As the second insulating film, a single layeror a stacked layer of a silicon oxide film, a silicon nitride film,and/or a silicon oxynitride (SiO_(x)N_(y)) film can be used.

As a portion of the second insulating film which is in contact with thesemiconductor layer, a silicon oxide film is preferably used. This isbecause the trap levels at the interface between the semiconductor layerand the second insulating film can be reduced.

A semiconductor layer is formed in a portion over the second insulatingfilm which overlaps with the first conductive layer by aphotolithography method, an inkjet method, a printing method, or thelike. A portion of the semiconductor layer extends to a portion in whichthe second insulating film and the first conductive layer are notoverlapped and which is over the second insulating film. Thesemiconductor layer includes a channel region (a channel region 110510),an LDD region (an LDD region 110508, an LDD region 110509), and animpurity region (an impurity region 110505, an impurity region 110506,an impurity region 110507). The channel region 110510 serves as achannel region of the transistor 110520. The LDD regions 110508 and110509 serve as LDD regions of the transistor 110520. Note that the LDDregions 110508 and 110509 are not necessarily formed. The impurityregion 110505 includes a portion which acts as one of a source electrodeand a drain electrode of the transistor 110520. The impurity region110506 includes a portion which acts as the other one of a sourceelectrode and a drain electrode of the transistor 110520. The impurityregion 110507 includes a portion which acts as a second electrode of thecapacitor 110521.

A third insulating film (an insulating film 110511) is formed entirely.A contact hole is selectively formed in part of the third insulatingfilm. The insulating film 110511 has a function of an interlayerinsulating film. As the third insulating film, an inorganic material(e.g., silicon oxide (SiO_(x)), silicon nitride, or silicon oxynitride),an organic compound material having a low dielectric constant (e.g., aphotosensitive or nonphotosensitive organic resin material), or the likecan be used. Alternatively, a material including siloxane may be used.Siloxane is a material in which a skeleton structure is formed by a bondof silicon (Si) and oxygen (O). As a substituent, an organic groupincluding at least hydrogen (e.g., an alkyl group or an aryl group) isused. As the substituent, a fluoro group may also be used.Alternatively, the organic group including at least hydrogen and thefluoro group may be used as the substituent.

A second conductive layer (a conductive layer 110512 and a conductivelayer 110513) is formed over the third insulating film. The conductivelayer 110512 is connected to the other of the source electrode and thedrain electrode of the transistor 110520 through the contact hole formedin the third insulating film. Therefore, the conductive layer 110512includes a portion which acts as the other one of the source electrodeand the drain electrode of the transistor 110520. When the conductivelayer 110513 is electrically connected to the conductive layer 110504,the conductive layer 110513 includes a portion which acts as a firstelectrode of the capacitor 110521. Alternatively, when the conductivelayer 110513 is electrically connected to the impurity region 110507,the conductive layer 110513 includes a portion which acts as a secondelectrode of the capacitor 110521. Alternatively, when the conductivelayer 110513 is not connected to the conductive layer 110504 and theimpurity region 110507, another capacitor is formed other than thecapacitor 110521. In this capacitor, the conductive layer 110513, theimpurity region 110507, and the insulating layer 110511 are used as afirst electrode, a second electrode, and an insulating layer,respectively. Note that as the second conductive layer, Ti, Mo, Ta, Cr,W, Al, Nd, Cu, Ag, Au, Pt, Nb, Si, Zn, Fe, Ba, Ge or an alloy of theseelements can be used. Further, a stacked layer including any of these(including an alloy thereof) can be used.

In steps after forming the second conductive layer, various insulatingfilms or various conductive films may be formed.

Next, structure of a transistor using amorphous silicon (a-Si:H) ormicrocrystal silicon as a semiconductor layer of the transistor and acapacitor are described.

FIG. 47 illustrates cross-sectional structures of a top gate transistorand a capacitor.

A first insulating film (an insulating film 110202) is formed entirelyover a substrate 110201. The first insulating film can preventimpurities from the substrate from adversely affecting a semiconductorlayer and changing a property of a transistor. In other words, the firstinsulating film serves as a base film. Therefore, a highly reliabletransistor can be manufactured. As the first insulating film, a singlelayer or a stacked layer of a silicon oxide film, a silicon nitridefilm, and/or a silicon oxynitride (SiO_(x)N_(y)) film can be used.

The first insulating film is not necessarily formed. If the firstinsulating film is not formed, the number of steps can be reduced, andthe manufacturing cost can be reduced. Since the structure can besimplified, the yield can be increased.

A first conductive layer (a conductive layer 110203, a conductive layer110204, and a conductive layer 110205) is formed over the firstinsulating film. The conductive layer 110203 includes a portion whichacts as one of a source electrode and a drain electrode of a transistor110220. The conductive layer 110204 includes a portion which acts as theother one of a source electrode and a drain electrode of the transistor110220. The conductive layer 110205 includes a portion which acts as afirst electrode of a capacitor 110221. As the first conductive layer,Ti, Mo, Ta, Cr, W, Al, Nd, Cu, Ag, Au, Pt, Nb, Si, Zn, Fe, Ba, Ge or analloy of these elements can be used. Further, a stacked layer includingany of these (including an alloy thereof) can be used.

Over the conductive layer 110203 and the conductive layer 110204, afirst semiconductor layer (a semiconductor layer 110206 and asemiconductor layer 110207) is formed. The semiconductor layer 110206includes a portion which acts as one of a source region and a drainregion. The semiconductor layer 110207 includes a portion which acts asthe other one of the source region and the drain region. As the firstsemiconductor layer, silicon including phosphorus or the like can beused.

A second semiconductor layer (a semiconductor layer 110208) is formed ina portion which is between the conductive layer 110203 and theconductive layer 110204 and over the first insulating film. A part ofthe semiconductor layer 110208 extends to a portion over the conductivelayer 110203 and the conductive layer 110204. The semiconductor layer110208 includes a portion which acts as a channel region of thetransistor 110220. As the second semiconductor layer, a semiconductorlayer having non-crystallinity such as amorphous silicon (a-Si:H), or asemiconductor layer such as microcrystal semiconductor (μ-Si:H) can beused.

A second insulating film (an insulating film 110209 and an insulatingfilm 110210) is formed to cover at least the semiconductor layer 110208and the conductive layer 110205. The second insulating film serves as agate insulating film. As the second insulating film, a single layer or astacked layer of a silicon oxide film, a silicon nitride film, and/or asilicon oxynitride (SiO_(x)N_(y)) film can be used.

As the second insulating film which is in contact with the secondsemiconductor layer, a silicon oxide film is preferably used. This isbecause the trap levels at the interface between the secondsemiconductor layer and the second insulating film can be reduced.

A second conductive layer (a conductive layer 110211 and a conductivelayer 110212) is formed over the second insulating film. The conductivelayer 110211 includes a portion which acts as a gate electrode of thetransistor 110220. The conductive layer 110212 serves as a secondelectrode or a wiring of the capacitor 110221. As the second conductivelayer, Ti, Mo, Ta, Cr, W, Al, Nd, Cu, Ag, Au, Pt, Nb, Si, Zn, Fe, Ba,Ge, or an alloy of these elements can be used. Further, a stacked layerincluding any of these (including an alloy thereof) can be used.

In steps after forming the second conductive layer, various insulatingfilms or various conductive films may be formed.

FIG. 48 illustrates cross-sectional structures of an inverted staggered(bottom gate) transistor and a capacitor. In particular, the transistorillustrated in FIG. 48 is a channel etch type transistor.

A first insulating film (an insulating film 110302) is formed entirelyover a substrate 110301. The first insulating film can preventimpurities from the substrate from adversely affecting a semiconductorlayer and changing a property of the transistor. In other words, thefirst insulating film serves as a base film. Therefore, a highlyreliable transistor can be manufactured. As the first insulating film, asingle layer or a stacked layer of a silicon oxide film, a siliconnitride film, and/or a silicon oxynitride film (SiO_(x)N_(y)) can beused.

The first insulating film is not necessarily formed. If the firstinsulating film is not formed, the number of steps can be reduced, andthe manufacturing cost can be reduced. Since the structure can besimplified, the yield can be increased.

A first conductive layer (a conductive layer 110303 and a conductivelayer 110304) is formed over the first insulating film. The conductivelayer 110303 includes a portion which acts as a gate electrode of atransistor 110320. The conductive layer 110304 includes a portion whichacts as a first electrode of a capacitor 110321. As the first conductivelayer, Ti, Mo, Ta, Cr, W, Al, Nd, Cu, Ag, Au, Pt, Nb, Si, Zn, Fe, Ba,Ge, or an alloy of these elements can be used. Further, a stacked layerincluding any of these (including an alloy thereof) can be used.

A second insulating film (an insulating film 110305) is formed to coverat least the first conductive layer. The second insulating film servesalso as a gate insulating film. As the second insulating film, a singlelayer or a stacked layer of a silicon oxide film, a silicon nitridefilm, and/or a silicon oxynitride (SiO_(x)N_(y)) film can be used.

As the second insulating film which is in contact with the semiconductorlayer, a silicon oxide film is preferably used. This is because the traplevels at the interface between the semiconductor layer and the secondinsulating film can be reduced.

A first semiconductor layer (a semiconductor layer 110306) is formed ina portion over the second insulating film which overlaps with the firstconductive layer by a photolithography method, an inkjet method, aprinting method, or the like. A portion of the semiconductor layer110306 extends to a portion in which the second insulating film and thefirst conductive layer are not overlapped. The semiconductor layer110306 includes a portion which acts as a channel region of thetransistor 110320. As the semiconductor layer 110306, a semiconductorlayer having non-crystallinity such as amorphous silicon (a-Si:H), or asemiconductor layer such as microcrystal semiconductor (μ-Si:H) can beused.

In a portion over the first semiconductor layer, a second semiconductorlayer (a semiconductor layer 110307 and a semiconductor layer 110308) isformed. The semiconductor layer 110307 includes a portion which acts asone of a source region and a drain region. The semiconductor layer110308 includes a portion which acts as the other one of the sourceregion and the drain region. As the second semiconductor layer, siliconincluding phosphorus or the like can be used.

A second conductive layer (a conductive layer 110309, a conductive layer110310, and a conductive layer 110311) is formed over the secondsemiconductor layer and the second insulating film. The conductive layer110309 includes a portion which acts as one of a source electrode and adrain electrode of the transistor 110320. The conductive layer 110310includes a portion which acts as the other one of the source electrodeand the drain electrode of the transistor 110320. The conductive layer110311 includes a portion which acts as a second electrode of thecapacitor 110321. Note that as the second conductive layer, Ti, Mo, Ta,Cr, W, Al, Nd, Cu, Ag, Au, Pt, Nb, Si, Zn, Fe, Ba, Ge, or an alloy ofthese elements can be used. Further, a stacked layer including any ofthese (including an alloy thereof) can be used.

In steps after forming the second conductive layer, various insulatingfilms or various conductive films may be formed.

A process of forming a channel etch type transistor is described as anexample. The first semiconductor layer and the second semiconductorlayer can be formed using the same mask. Specifically, the firstsemiconductor layer and the second semiconductor layer are formedsequentially. The first semiconductor layer and the second semiconductorlayer are formed using the same mask.

A process of forming a channel etch type transistor is described asanother example. Without using a new mask, a channel region of atransistor can be formed. Specifically, after forming the secondconductive layer, a part of the second semiconductor layer is removedusing the second conductive layer as a mask. Alternatively, a portion ofthe second semiconductor layer is removed by using the same mask as thesecond conductive layer. The first semiconductor layer below the removedsecond semiconductor layer becomes a channel region of the transistor.

FIG. 49 illustrates cross-sectional structures of an inverted staggered(a bottom gate) transistor and a capacitor. In particular, thetransistor illustrated in FIG. 49 is a channel protection (channel stop)type transistor.

A first insulating film (an insulating film 110402) is formed entirelyover a substrate 110401. The first insulating film can preventimpurities from the substrate from adversely affecting a semiconductorlayer and changing a property of a transistor. In other words, the firstinsulating film serves as a base film. Therefore, a highly reliabletransistor can be manufactured. As the first insulating film, a singlelayer or a stacked layer of a silicon oxide film, a silicon nitridefilm, and/or a silicon oxynitride (SiO_(x)N_(y)) film can be used.

The first insulating film is not necessarily formed. If the firstinsulating film is not formed, the number of steps can be reduced, andthe manufacturing cost can be reduced. Since the structure can besimplified, the yield can be increased.

A first conductive layer (a conductive layer 110403 and a conductivelayer 110404) is formed over the first insulating film. The conductivelayer 110403 includes a portion which acts as a gate electrode of atransistor 110420. The conductive layer 110404 includes a portion whichacts as a first electrode of a capacitor 110421. As the first conductivelayer, Ti, Mo, Ta, Cr, W, Al, Nd, Cu, Ag, Au, Pt, Nb, Si, Zn, Fe, Ba,Ge, or an alloy of these elements can be used. Further, a stacked layerincluding any of these (including an alloy thereof) can be used.

A second insulating film (an insulating film 110405) is formed to coverat least the first conductive layer. The second insulating film servesas a gate insulating film. As the second insulating film, a single layeror a stacked layer of a silicon oxide film, a silicon nitride film,and/or a silicon oxynitride (SiO_(x)N_(y)) film can be used.

As the second insulating film which is in contact with the semiconductorlayer, a silicon oxide film is preferably used. This is because the traplevels at the interface between the semiconductor layer and the secondinsulating film can be reduced.

A first semiconductor layer (a semiconductor layer 110406) is formed ina portion over the second insulating film which overlaps with the firstconductive layer, by a photolithography method, an inkjet method, aprinting method, or the like. A portion of the semiconductor layer110406 extends to a portion in which the second insulating film and thefirst conductive layer are not overlapped. The semiconductor layer110406 includes a portion which acts as a channel region of thetransistor 110420. As the semiconductor layer 110406, a semiconductorlayer having non-crystallinity such as amorphous silicon, or asemiconductor layer such as microcrystal semiconductor can be used.

A third insulating film (an insulating film 110412) is formed in aportion over the first semiconductor layer. The insulating film 110412has a function of preventing the channel region of the transistor 110420from being etched. In other words, the insulating film 110412 serves asa channel protection film (channel stop film). As the third insulatingfilm, a single layer or a stacked layer of a silicon oxide film, asilicon nitride film, and/or a silicon oxynitride (SiO_(x)N_(y)) filmcan be used.

In a portion over the first semiconductor layer and a portion over thethird insulating film, a second semiconductor layer (a semiconductorlayer 110407 and a semiconductor layer 110408) is formed. Thesemiconductor layer 110407 includes a portion which acts as one of asource region and a drain region. The semiconductor layer 110408includes a portion which acts as the other one of the source region andthe drain region. As the second semiconductor layer, silicon includingphosphorus or the like can be used.

A second conductive layer (a conductive layer 110409, a conductive layer110410, and a conductive layer 110411) is formed over the secondsemiconductor layer. The conductive layer 110409 includes a portionwhich acts as one of a source electrode and a drain electrode of thetransistor 110420. The conductive layer 110410 includes a portion whichacts as the other one of the source electrode and the drain electrode ofthe transistor 110420. The conductive layer 110411 includes a portionwhich acts as a second electrode of the capacitor 110421. Note that asthe second conductive layer, Ti, Mo, Ta, Cr, W, Al, Nd, Cu, Ag, Au, Pt,Nb, Si, Zn, Fe, Ba, Ge, or an alloy of these elements can be used.Further, a stacked layer including any of these (including an alloythereof) can be used.

In steps after forming the second conductive layer, various insulatingfilms or various conductive films may be formed.

The structures and manufacturing methods of transistors have beendescribed above. Such wirings, electrodes, conductive layers, conductivefilms, terminals, vias, plugs, and the like are formed using one or moreelements selected from the group consisting of aluminum (Al), tantalum(Ta), titanium (Ti), molybdenum (Mo), tungsten (W), neodymium (Nd),chromium (Cr), nickel (Ni), platinum (Pt), gold (Au), silver (Ag),copper (Cu), magnesium (Mg), scandium (Sc), cobalt (Co), zinc (Zn),niobium (Nb), silicon (Si), phosphorus (P), boron (B), arsenic (As),gallium (Ga), indium (In), tin (Sn), and oxygen (O); a compound or analloy material including one or more of the elements in the group (forexample, indium tin oxide (ITO), indium zinc oxide (IZO), indium tinoxide including silicon oxide (ITSO), zinc oxide (ZnO), tin oxide (SnO),cadmium tin oxide, aluminum neodymium (Al—Nd), magnesium silver (Mg—Ag),molybdenum-niobium (Mo—Nb), or the like); a substance in which thesecompounds are combined; or the like. Alternatively, such wirings,electrodes, conductive layers, conductive films, terminals, vias, plugs,and the like are preferably formed using a substance including suchcompounds, a compound of silicon and one or more of the elementsselected from the group (silicide) (e.g., aluminum silicon, molybdenumsilicon, nickel silicide); or a compound of nitrogen and one or more ofthe elements selected from the group (e.g., titanium nitride, tantalumnitride, molybdenum nitride).

Note that silicon (Si) may include an n-type impurity (such asphosphorus) or a p-type impurity (such as boron). The impurity containedin silicon can increase the conductivity or enables the same performanceas normal conductors. Thus, such silicon can be utilized easily aswirings or electrodes.

Silicon can be any of various types of silicon such as single crystalsilicon, polycrystal silicon, or microcrystal silicon. Alternatively,silicon having no crystallinity such as amorphous silicon can be used.By using single crystal silicon or polycrystal silicon, resistance of awiring, an electrode, a conductive layer, a conductive film, or aterminal can be reduced. By using amorphous silicon or microcrystallinesilicon, a wiring or the like can be formed by a simple process.

In addition, aluminum or silver has high conductivity, and thus canreduce a signal delay. Since aluminum or silver can be easily etched,aluminum or silver can be easily patterned and processed minutely.

Further, copper also has high conductivity, and thus can reduce a signaldelay. In using copper, a stacked structure is preferably employed toenhance adhesiveness.

Molybdenum and titanium are also preferable materials. This is becauseeven if molybdenum or titanium is in contact with an oxide of asemiconductor (ITO, IZO, or the like) or silicon, molybdenum or titaniumdoes not cause defects. Further, molybdenum or titanium is easily etchedand has high-heat resistance.

Tungsten is preferable since tungsten has high-heat resistance.

Neodymium is also preferable, since neodymium has an advantage of highheat resistance. In particular, an alloy of neodymium and aluminum isused to increase heat resistance, thereby almost preventing hillocks ofaluminum.

Moreover, silicon is preferable since silicon can be formed at the sametime as a semiconductor layer included in a transistor, and hashigh-heat resistance.

Since ITO, IZO, ITSO, zinc oxide (ZnO), silicon (Si), tin oxide (SnO),and cadmium tin oxide have light-transmitting properties, they can beused as a portion which allows light to pass through. For example, ITO,IZO, ITSO, zinc oxide (ZnO), silicon (Si), tin oxide (SnO), or cadmiumtin oxide can be used for a pixel electrode and/or a common electrode.

Note that IZO is preferable since IZO is easily etched and processed. Inetching IZO, almost no residues of IZO are left. Thus, when a pixelelectrode is formed using IZO, defects (such as short-circuiting ororientation disorder) of a liquid crystal element or a light-emittingelement can be reduced.

Such wirings, electrodes, conductive layers, conductive films,terminals, vias, plugs, or the like may have a single-layer structure ora multilayer structure. By adopting a single-layer structure, amanufacturing process of such wirings, electrodes, conductive layers,conductive films, or terminals can be simplified; the number of days fora process can be reduced; and cost can be reduced. Alternatively, byemploying a multilayer structure, an advantage of each material is takenand a disadvantage thereof is reduced so that a wiring, an electrode, orthe like with high performance can be formed. For example, alow-resistant material (such as aluminum) is included in a multilayerstructure, thereby reducing the resistance of such wirings. As anotherexample, when a low heat-resistant material is interposed between highheat-resistant materials to form a stacked-layer structure, heatresistance of wirings, electrodes, or the like can be increased,utilizing advantages of such low heat-resistance materials. For example,a layer including aluminum is preferably interposed between layersincluding molybdenum, titanium, or neodymium as a stacked structure.

If wirings or electrodes are in direct contact with each other, anadverse effect is caused to each other in some cases. For example, oneof wirings or electrodes is mixed into a material of the other of thewirings or electrodes and changes the property, and thus, a desiredfunction cannot be obtained. As another example, in forming ahigh-resistant portion, there is a problem in that it cannot be formednormally. In such a case, a reactive material is preferably sandwichedby or covered with a non-reactive material in a stacked structure. Forexample, when ITO is connected to aluminum, an alloy of titanium,molybdenum, and neodymium is preferably disposed between the ITO and thealuminum. As another example, when silicon is connected to aluminum, analloy of titanium, molybdenum, and neodymium is preferably disposedbetween the silicon and the aluminum.

Note that the term “wiring” indicates a portion including a conductor.The shape of such a wiring may be linear; but not limited to, such awiring may be short. Therefore, electrodes are included in such wirings.

Note that a carbon nanotube may be used for wirings, electrodes,conductive layers, conductive films, terminals, vias, plugs, or thelike. Since the carbon nanotube has a light-transmitting property, itcan be used for a portion which allows light to pass thorough. Forexample, the carbon nanotube can be used for a pixel electrode and/or acommon electrode.

Although this embodiment mode has been described with reference tovarious drawings, the contents (or part of the contents) described ineach drawing can be freely applied to, combined with, or replaced withthe contents (or part of the contents) described in another drawing.Further, much more drawings can be formed by combining each part in theabove-described drawings with another part.

Similarly, the contents (or part of the contents) described in eachdrawing in this embodiment mode can be freely applied to, combined with,or replaced with the contents (or part of the contents) described in adrawing in another embodiment mode. Further, much more drawings can beformed by combining each part in the drawings in this embodiment modewith part of another embodiment mode.

Note that this embodiment mode has described just examples of embodying,slightly transforming, modifying, improving, describing in detail, orapplying the contents (or part of the contents) described in otherembodiment modes, an example of related part thereof, or the like.Therefore, the contents described in other embodiment modes can befreely applied to, combined with, or replaced with the contentsdescribed in this embodiment mode.

Embodiment Mode 9

Embodiment Mode 9 will describe a configuration of a display device.

FIG. 53A illustrates a configuration of a display device. FIG. 53A is atop view of the display device.

A pixel portion 170101, a scan line side input terminal 170103, and asignal line side input terminal 170104 are formed over a substrate170100, scan lines extend in a row direction from the scan line sideinput terminal 170103, and signal lines extend in a column directionfrom the signal line side input terminal 170104 over the substrate170100. Pixels 170102 are disposed in matrix and each pixel 170102 isdisposed at an intersection portion of the scan line and the signal linein the pixel portion 170101.

The case in which signals are input from an external driver circuit hasbeen described above. However, the present invention is not limited tothis case, and an IC chip can be mounted on a display device.

For example, as shown in FIG. 54A, an IC chip 170201 can be mounted on asubstrate 170100 by a COG (chip on glass) method. In this case,inspection can be conducted before mounting the IC chip 170201 on thesubstrate 170100 to increase the yield of the display device. Further,the reliability can also be increased. In addition, the same portions asthose in FIG. 53A are denoted by the same reference numerals and thedescription thereof is omitted.

As another example, as shown in FIG. 54B, an IC chip 170201 can bemounted on an FPC (flexible printed circuit) 170200 by a TAB (tapeautomated bonding) method. In this case, inspection can be conductedbefore mounting the IC chip 170201 on the FPC 170200 to increase theyield of the display device. Further, the reliability can also beincreased. In addition, the same portions as those in FIG. 53A aredenoted by the same reference numerals and the description thereof isomitted.

As well as the IC chip can be mounted on the substrate 170100, a drivercircuit can be formed on the substrate 170100.

For example, as shown in FIG. 53B, a scan line driver circuit 170105 canbe formed on a substrate 170100. In this case, the number of componentscan be reduced to decrease the manufacturing cost. The number ofconnection points with circuit components can be reduced to enhance thereliability. Since the driving frequency of the scan line driver circuit170105 is low, the scan line driver circuit 170105 can be easily formedusing amorphous silicon or microcrystal silicon as a semiconductor layerof a transistor. In addition, an IC chip for outputting a signal to thesignal line may be mounted on the substrate 170100 by a COG method.Alternatively, an FPC on which an IC chip for outputting a signal to asignal line is mounted by a TAB method may be disposed on the substrate170100. In addition, an IC chip for controlling the scan line drivercircuit 170105 may be mounted on the substrate 170100 by a COG method.Alternatively, an FPC on which an IC chip for controlling the scan linedriver circuit 170105 is mounted by a TAB method may be disposed on thesubstrate 170100. In addition, the same portions as those in FIG. 53Aare denoted by the same reference numerals and the description thereofis omitted.

As another example, as shown in FIG. 53C, the scan line driver circuit170105 and a signal line driver circuit 170106 can be formed on thesubstrate 170100. Thus, the number of components can be reduced todecrease the manufacturing cost. The number of connection points withcircuit components can be reduced to enhance the reliability. Inaddition, an IC chip for controlling the scan line driver circuit 170105may be mounted on the substrate 170100 by a COG method. Alternatively,an FPC on which an IC chip for controlling the scan line driver circuit170105 is mounted by a TAB method may be disposed on the substrate170100. An IC chip for controlling the signal line driver circuit 170106may be mounted on the substrate 170100 by a COG method. Alternatively,an IC chip for controlling the signal line driver circuit 170106 may bemounted on the substrate 170100 by a TAB method. In addition, the sameportions as those in FIG. 53A are denoted by the same reference numeralsand the description thereof is omitted.

Although this embodiment mode has been described with reference tovarious drawings, the contents (or part of the contents) described ineach drawing can be freely applied to, combined with, or replaced withthe contents (or part of the contents) described in another drawing.Further, much more drawings can be formed by combining each part in theabove-described drawings with another part.

Similarly, the contents (or part of the contents) described in eachdrawing in this embodiment mode can be freely applied to, combined with,or replaced with the contents (or part of the contents) described in adrawing in another embodiment mode. Further, much more drawings can beformed by combining each part in the drawings in this embodiment modewith part of another embodiment mode.

Note that this embodiment mode has described just examples of embodying,slightly transforming, modifying, improving, describing in detail, orapplying the contents (or part of the contents) described in otherembodiment modes, an example of related part thereof, or the like.Therefore, the contents described in other embodiment modes can befreely applied to, combined with, or replaced with the contentsdescribed in this embodiment mode.

Embodiment Mode 10

Embodiment Mode 10 will describe a method for driving a display device.In particular, a method for driving a liquid crystal display device isdescribed.

A liquid crystal display panel which can be used for the liquid crystaldisplay device described in this embodiment mode has a structure inwhich a liquid crystal material is sandwiched between two substrates.Each of the two substrates is provided with an electrode for controllingan electric field applied to the liquid crystal material. A liquidcrystal material corresponds to a material optical and electricalproperties of which are changed by an electric field applied fromoutside. Therefore, a liquid crystal panel corresponds to a device inwhich desired optical and electrical properties can be obtained bycontrolling voltage applied to the liquid crystal material using theelectrode provided for each of the two substrates. In addition, aplurality of electrodes are disposed in a planar manner, each of theelectrodes corresponds to a pixel, and voltages applied to the pixelsare individually controlled. Therefore, a liquid crystal display panelwhich can display a clear image can be obtained.

Here, response time of the liquid crystal material with respect to achange in electric field depends on a gap between the two substrates (acell gap) and a type of the liquid crystal material, and is generallyseveral milliseconds to several ten milliseconds. Further, in the casewhere the amount of change in electric field is small, the response timeof the liquid crystal material is further lengthened. Thischaracteristic causes a defect in image display such as an after image,persistence of vision, or decrease in contrast when the liquid crystalpanel displays a moving image. In particular, when a half tone ischanged into another half tone (change in electric field is small), theabove-described defect becomes noticeable.

Meanwhile, as a particular problem of a liquid crystal panel using anactive matrix method, fluctuation in writing voltage due to constantelectric charge driving is given. Constant electric charge driving inthis embodiment mode is described below.

A pixel circuit using an active matrix method includes a switch whichcontrols writing and a capacitor which holds an electric charge. Amethod for driving the pixel circuit using the active matrix methodcorresponds to a method in which predetermined voltage is written in apixel circuit with a switch turned on, and the switch is turned offimmediately after that, and an electric charge in the pixel circuit isheld (a hold state). At the time of hold state, exchange of the electriccharges between inside and outside of the pixel circuit is not performed(a constant electric charge). Usually, the length of a period duringwhich the switch is turned off is approximately several hundreds times(by the number of scan lines) longer than that of a period during whichthe switch is turned on. Therefore, it may be considered that the switchof the pixel circuit be almost always turned off. As described above,constant electric charge driving in this embodiment mode corresponds toa driving method in which a pixel circuit is in a hold state in almostall periods in driving a liquid crystal panel.

Next, electrical properties of the liquid crystal material aredescribed. The liquid crystal material changes its dielectric constantas well as optical properties when an electric field applied fromoutside is changed. That is, when each pixel of the liquid crystal panelis regarded as a capacitor (a liquid crystal element) sandwiched betweentwo electrodes, the capacitor corresponds to a capacitor which changesits capacitance in accordance with voltage applied. This phenomenon iscalled dynamic capacitance.

When a capacitor which changes its capacitance in accordance withvoltage applied in this manner is driven by constant electric chargedriving, the following problem occurs. If capacitance of a liquidcrystal element is changed in a hold state in which an electric chargeis not transferred, voltage to be applied is also changed. This can beunderstood from the fact that the amount of electric charges is constantin a relational expression of (the amount of electriccharges)=(capacitance)×(applied voltage).

For the above reasons, voltage at the time of a hold state is changedfrom voltage at the time of writing because constant electric chargedriving is performed in a liquid crystal panel using an active matrixmethod. Accordingly, change in transmittivity of the liquid crystalelement is different from change in transmittivity of a liquid crystalelement in a driving method which does not take a hold state. FIGS. 51Ato 51C show this state. FIG. 51A illustrates an example of controllingvoltage written in a pixel circuit in the case where time is representedby the horizontal axis and the absolute value of the voltage isrepresented by the vertical axis. FIG. 51B illustrates an example ofcontrolling voltage written in the pixel circuit in the case where timeis represented by the horizontal axis and the voltage is represented bythe vertical axis. FIG. 51C illustrates a change in transmittivity ofthe liquid crystal element over time in the case where the voltage shownin FIG. 51A or 51B is written in the pixel circuit when time isrepresented by the horizontal axis and the transmittivity of the liquidcrystal element is represented by the vertical axis. In each of FIGS.51A to 51C, a period F refers to a period for rewriting the voltage, andtime for rewriting the voltage is denoted by t₁, t₂, t₃, and t₄.

Here, writing voltage corresponding to image data input to the liquidcrystal display device corresponds to |V₁| in rewriting at the time of 0and corresponds to |V₂| in rewriting at the time of t₁, t₂, t₃, and t₄(see FIG. 51A).

Note that polarity of the writing voltage corresponding to image datainput to the liquid crystal display device may be switched periodically(inversion driving: see FIG. 51B). Since DC voltage can be preventedfrom being applied to a liquid crystal as much as possible by using thismethod, burn-in or the like caused by deterioration of the liquidcrystal element can be prevented. Note also that a period of switchingthe polarity (an inversion period) may be the same as a period ofrewriting voltage. In this case, generation of flickers caused byinversion driving can be reduced because the inversion period is short.Further, the inversion period may be a period which is integral times ofthe period of rewriting voltage. In this case, power consumption can bereduced because the inversion period is long and the frequency ofchanging the polarity and that of writing voltage can be decreased.

FIG. 51C illustrates a change in transmittivity of the liquid crystalelement over time in the case where voltage as shown in FIG. 51A or 51Bis applied to the liquid crystal element. Here, transmittivity of theliquid crystal element after the voltage |V₁| is applied to the liquidcrystal element and after sufficient time passes corresponds to TR₁.Similarly, transmittivity of the liquid crystal element after thevoltage |V₂| is applied to the liquid crystal element and aftersufficient time passes corresponds to TR₂. When the voltage applied tothe liquid crystal element is changed from |V₁| to |V₂| at the time oft₁, transmittivity of the liquid crystal element does not immediatelybecome TR₂ as shown by a dashed line 30401 but slowly changes. Forexample, when the period of rewriting voltage is the same as a frameperiod of a video signal of 60 Hz (16.7 milliseconds), it takes aboutseveral frames for transmittivity to be changed to TR₂.

Note that a smooth change in transmittivity over time as shown in thedashed line 30401 corresponds to a change in transmittivity over timewhen the voltage |V₂| is accurately applied to the liquid crystalelement. In an actual liquid crystal panel, for example, a liquidcrystal panel using an active matrix method, transmittivity of theliquid crystal element does not change over time as shown by the dashedline 30401 but changes gradually over time as shown by a solid line30402 because voltage at the time of a hold state is changed fromvoltage at the time of writing due to constant electric charge driving.This is because the voltage is changed due to constant electric chargedriving, so that it is impossible to reach intended voltage only bysingle writing. Accordingly, the response time of the liquid crystalelement becomes further longer in appearance than original response time(the dashed line 30401), so that a noticeable defect in image displaysuch as an after image, persistence of vision, or decrease in contrastoccurs.

By using overdriving, it is possible to solve at the same time, problemsof the long length of original response time of the liquid crystalelement and the phenomenon in which the response time in appearancebecomes further longer because of shortage of writing by dynamiccapacitance and constant electric charge driving. FIGS. 52A to 52C showthis state. FIG. 52A illustrates an example of controlling voltagewritten in a pixel circuit in the case where time is represented by thehorizontal axis and the absolute value of the voltage is represented bythe vertical axis. FIG. 52B illustrates an example of controllingvoltage written in the pixel circuit in the case where time isrepresented by the horizontal axis and the voltage is represented by thevertical axis. FIG. 52C illustrates a change in transmittivity of theliquid crystal element over time in the case where the voltage shown inFIG. 52A or 52B is written in the pixel circuit when time is representedby the horizontal axis and the transmittivity of the liquid crystalelement is represented by the vertical axis. In each of FIGS. 52A to52C, a period F refers to a period for rewriting the voltage, and timefor rewriting the voltage is denoted by t₁, t₂, t₃, and t₄.

Here, writing voltage corresponding to image data input to the liquidcrystal display device corresponds to |V₁| in rewriting at the time of0, corresponds to |V₃| in rewriting at the time of t₁, and correspondsto |V₃| in writing at the time of t₂, t₃, and t₄ (see FIG. 52A).

Note that polarity of the writing voltage corresponding to image datainput to the liquid crystal display device may be switched periodically(inversion driving: see FIG. 52B). Since DC voltage can be preventedfrom being applied to a liquid crystal as much as possible by using thismethod, burn-in or the like caused by deterioration of the liquidcrystal element can be prevented. Note also that a cycle of switchingthe polarity (an inversion cycle) may be the same as a cycle ofrewriting voltage. In this case, generation of flickers caused byinversion driving can be reduced because the inversion period is short.Further, the inversion period may be a period which is integral times ofthe period of rewriting voltage. In this case, power consumption can bereduced because the inversion period is long and the frequency ofchanging the polarity and that of writing voltage can be decreased.

FIG. 52C illustrates a change in transmittivity of the liquid crystalelement over time in the case where voltage as shown in FIG. 52A or 52Bis applied to the liquid crystal element. Here, transmittivity of theliquid crystal element after the voltage |V₁| is applied to the liquidcrystal element and after sufficient time passes corresponds to TR₁.Similarly, transmittivity of the liquid crystal element after thevoltage |V₂| is applied to the liquid crystal element and aftersufficient time passes corresponds to TR₂. Similarly, transmittivity ofthe liquid crystal element after the voltage |V₃| is applied to theliquid crystal element and after sufficient time passes corresponds toTR₃. When the voltage applied to the liquid crystal element is changedfrom |V₁| to |V₃| at the time of t₁, transmittivity of the liquidcrystal element tends to be changed to TR₃ in several frames as shown bya dashed line 30501. However, application of the voltage |V₃| isterminated at the time t₂ and the voltage |V₂| is applied after the timet₂. Therefore, transmittivity of the liquid crystal element does notbecome as shown by the dashed line 30501 but becomes as shown by a solidline 30502. Here, it is preferable that a value of the voltage |V₃| beset so that transmittivity is approximately TR₂ at the time of t₂. Here,the voltage |V₃| is also referred to as overdriving voltage.

That is, the response time of the liquid crystal element can becontrolled to some extent by changing |V₃| which is the overdrivingvoltage. This is because the response time of the liquid crystal elementis changed in accordance with the intensity of an electric field.Specifically, the response time of the liquid crystal element becomesshorter as the electric field becomes stronger, and the response time ofthe liquid crystal element becomes longer as the electric field becomesweaker.

Note that it is preferable that the overdriving voltage |V₃| be changedin accordance with the amount of change in the voltage, i.e., thevoltage |V₁| and the voltage |V₂| which provide intendedtransmittivities TR₁ and TR₂. This is because optimum response time canbe always obtained by changing the overdriving voltage |V₃| inaccordance with change in the response time of the liquid crystalelement, even when the response time of the liquid crystal element ischanged by the amount of change in the voltage.

Note also that it is preferable that the overdriving voltage |V₃| bechanged in accordance with a mode of the liquid crystal element such asa TN-mode, a VA-mode, an IPS-mode, or an OCB-mode. This is becauseoptimum response time can be always obtained by changing the overdrivingvoltage |V₃| in accordance with change in the response time of theliquid crystal element, even when the response time of the liquidcrystal element varies depending on the mode of the liquid crystalelement.

Note also that the voltage rewriting period F may be the same as a frameperiod of an input signal. In this case, a liquid crystal display devicewith low manufacturing cost can be obtained because a peripheral drivercircuit of the liquid crystal display device can be simplified.

Note also that the voltage rewriting period F may be shorter than theframe period of the input signal. For example, the voltage rewritingperiod F may be a half (½) of the frame period of the input signal, onethird (⅓) of the frame period of the input signal, or shorter than onethird (⅓) of the frame period of the input signal. It is effective tocombine this method with a countermeasure against deterioration inquality of a moving image caused by hold driving of the liquid crystaldisplay device, such as black frame insertion driving, backlightblinking, backlight scanning, or intermediate image insertion driving bymotion compensation. That is, short response time of the liquid crystalelement is required for the countermeasure against deterioration inquality of a moving image caused by hold driving of the liquid crystaldisplay device, and the response time of the liquid crystal element canbe shortened relatively easily by using the overdriving described inthis embodiment mode. Although the response time of the liquid crystalelement can be essentially shortened by a cell gap, a liquid crystalmaterial, a mode of the liquid crystal element, or the like, it istechnically difficult to shorten the response time of the liquid crystalelement. Therefore, it is very important to use a method for shorteningthe response time of the liquid crystal element by a driving method suchas overdriving.

Note also that the voltage rewriting period F may be longer than theframe period of the input signal. For example, the voltage rewritingperiod F may be twice as long as the frame period of the input signal,three times as long as the frame period of the input signal, or longerthan three times. It is effective to combine this method with a unit (acircuit) which determines whether voltage is rewritten or not for a longperiod. That is, when the voltage is not rewritten for a long period, anoperation of the circuit can be stopped during the period, withoutperforming a rewriting operation itself of the voltage. Therefore, aliquid crystal display device which consumes less power can be obtained.

Next, a specific method for changing the overdriving voltage |V₃| inaccordance with the voltage |V₁₁ and the voltage |V₂| which provideintended transmittivity TR₁ and TR₂ is described.

An overdriving circuit corresponds to a circuit for appropriatelycontrolling the overdriving voltage |V₃| in accordance with the voltage|V₁ and the voltage |V₂| which provide intended transmittivity TR₁ andTR₂. Therefore, signals input to the overdriving circuit are a signalwhich is related to the voltage |V₁₁ which provides intendedtransmittivity TR₁ and a signal which is related to the voltage |V₂|which provides intended transmittivity TR₂, and a signal output from theoverdriving circuit is a signal which is related to the overdrivingvoltage |V₃₁. Here, each of these signals may have an analog voltagevalue such as the voltage applied to the liquid crystal element (e.g.,|V₁|, |V₂|, or |V₃|) or may be a digital signal for supplying thevoltage applied to the liquid crystal element. Here, description is maderegarding the signal which is related to the overdriving circuit as adigital signal.

First, an overall structure of the overdriving circuit is described withreference to FIG. 88A. Here, input video signals 30101 a and 30101 b areused as signals for controlling the overdriving voltage. As a result ofprocessing these signals, an output video signal 30104 is to be outputas a signal which supplies the overdriving voltage.

Here, since the voltage |V₁| and the voltage |V₂| which provide intendedtransmittivity TR₁ and TR₂ are video signals in adjacent frames, it ispreferable that the input video signals 30101 a and 30101 b be similarlyvideo signals in adjacent frames. In order to obtain such signals, theinput video signal 30101 a is input to a delay circuit 30102 in FIG. 88Aand a signal which is consequently output can be used as the input videosignal 30101 b. An example of the delay circuit 30102 is a memory. Thatis, the input video signal 30101 a is stored in the memory in order todelay the input video signal 30101 a for one frame; a signal stored inthe previous frame is taken out from the memory as the input videosignal 30101 b at the same time; and the input video signal 30101 a andthe input video signal 30101 b are concurrently input to a correctioncircuit 30103. Accordingly, the video signals in adjacent frames can behandled. By inputting the video signals in adjacent frames to thecorrection circuit 30103, the output video signal 30104 can be obtained.Note that a memory which can be used as the delay circuit 30102 in orderto delay the input video signal 30101 a for one frame is a memory havingcapacity for storing a video signal for one frame (i.e., a framememory). Thus, the memory can have a function as a delay circuit withoutexcess and deficiency of memory capacity.

Next, the delay circuit 30102 structured mainly for reducing memorycapacity is described. Since memory capacity can be reduced by usingsuch a circuit as the delay circuit 30102, manufacturing cost can bereduced.

Specifically, a delay circuit as shown in FIG. 88B can be used as thedelay circuit 30102 having such characteristics. The delay circuit 30102shown in FIG. 88B includes an encoder 30105, a memory 30106, and adecoder 30107.

Operations of the delay circuit 30102 shown in FIG. 88B are as follows.First, the encoder 30105 performs compression processing before theinput video signal 30101 a is stored in the memory 30106. Thus, the sizeof data to be stored in the memory 30106 can be reduced. Accordingly,memory capacity can be reduced, so that manufacturing cost can also bereduced. Then, a compressed video signal is transferred to the decoder30107, where decompression processing is performed. Thus, the previoussignal which has been compressed by the encoder 30105 can be restored.Here, compression/decompression processing which is performed by theencoder 30105 and the decoder 30107 may be reversible processing. Thus,since the video signal does not deteriorate even aftercompression/decompression processing is performed, memory capacity canbe reduced without causing deterioration of the quality of an image tobe finally displayed by a device. Further, compression/decompressionprocessing which is performed by the encoder 30105 and the decoder 30107may be irreversible processing. Thus, since the data size of thecompressed video signal can be made extremely small, memory capacity canbe significantly reduced.

Note that as a method for reducing memory capacity, various methods canbe used as well as the above-described method. A method in which colorinformation included in a video signal is reduced (e.g., color reductionfrom 2.6 hundred thousand colors to 65 thousand colors is performed) orthe number of data is reduced (e.g., resolution is decreased) withoutperforming image compression by an encoder, or the like can be used.

Next, specific examples of the correction circuit 30103 are describedwith reference to FIGS. 88C to 88E. The correction circuit 30103corresponds to a circuit for outputting an output video signal having acertain value from two input video signals. Here, when the relationshipbetween the two input video signals and the output video signal isnon-linear and it is difficult to calculate the relationship by simpleoperation, a look-up table (an LUT) may be used as the correctioncircuit 30103. Since the relationship between the two input videosignals and the output video signal is calculated in advance bymeasurement for an LUT, the output video signal corresponding to the twoinput video signals can be calculated only by referring to the LUT (seeFIG. 88C). By using a LUT 30108 as the correction circuit 30103, thecorrection circuit 30103 can be realized without performing complicatedcircuit design or the like.

Here, since the LUT 30108 is one of memories, it is preferable to reducememory capacity as much as possible in order to reduce manufacturingcost. A possible example of the correction circuit 30103 for realizingreduction in memory capacity is a circuit shown in FIG. 88D. Thecorrection circuit 30103 shown in FIG. 88D includes an LUT 30109 and anadder 30110. The LUT 30109 stores differential data between the inputvideo signal 30101 a and the output video signal 30104 to be output.That is, the output video signal 30104 can be obtained by taking outcorresponding differential data from the LUT 30109 based on the inputvideo signal 30101 a and the input video signal 30101 b and adding thetaken differential data and the input video signal 30101 a by the adder30110. Note that when data stored in the LUT 30109 is differential data,memory capacity of the LUT 30109 can be reduced. This is because datasize of differential data is smaller than that of the output videosignal 30104, so that memory capacity necessary for the LUT 30109 can bedecreased.

In addition, when the output video signal can be calculated by simpleoperation such as four arithmetic operations of the two input videosignals, the correction circuit 30103 can be realized by combination ofsimple circuits such as an adder, a subtracter, or a multiplier.Accordingly, it becomes unnecessary to use an LUT, so that manufacturingcost can be significantly reduced. An example of such a circuit is acircuit shown in FIG. 88E. The correction circuit 30103 shown in FIG.88E includes a subtracter 30111, a multiplier 30112, and an adder 30113.First, difference between the input video signal 30101 a and the inputvideo signal 30101 b is calculated by the subtracter 30111. After that,a differential value is multiplied by an appropriate coefficient byusing the multiplier 10112. Then, by adding the differential valuemultiplied by the appropriate coefficient to the input video signal30101 a by the adder 30113, the output video signal 30104 can beobtained. By using such a circuit, it becomes unnecessary to use theLUT. Therefore, manufacturing cost can be significantly reduced.

Note that by using the correction circuit 30103 shown in FIG. 88E undera certain condition, inappropriate output of the output video signal30104 can be prevented. The condition is that the output video signal30104 which supplies the overdriving voltage and a differential valuebetween the input video signals 30101 a and 30101 b have linearity. Inaddition, the slope of this linearity corresponds to a coefficient to bemultiplied by the multiplier 30112. That is, it is preferable that thecorrection circuit 30103 shown in FIG. 88E be used for a liquid crystalelement having such a property. An example of a liquid crystal elementhaving such a property is an IPS-mode liquid crystal element in whichresponse time has little grayscale dependency. For example, by using thecorrection circuit 30103 shown in FIG. 88E for an IPS-mode liquidcrystal element in this manner, manufacturing cost can be significantlyreduced and an overdriving circuit which can prevent output of theinappropriate output video signal 30104 can be provided.

Operations which are similar to those of the circuits shown in FIGS. 88Ato 88E may be realized by software processing. As for the memory usedfor the delay circuit, another memory included in the liquid crystaldisplay device, a memory included in a device which transmits an imageto be displayed on the liquid crystal display device (e.g., a video cardor the like included in a personal computer or a device equivalent tothe personal computer) can be used for example. Thus, not only canmanufacturing cost be reduced, but the extent of overdriving, useconditions, or the like can be selected in accordance with user'spreference.

Next, driving for controlling a potential of a common line is describedwith reference to FIGS. 89A and 89B. FIG. 89A illustrates a plurality ofpixel circuits in which one common line is provided with respect to onescan line in a display device using a display element which hascapacitive properties like a liquid crystal element. Each of the pixelcircuits shown in FIG. 89A includes a transistor 30201, an auxiliarycapacitor 30202, a display element 30203, a video signal line 30204, ascan line 30205, and a common line 30206.

A gate electrode of the transistor 30201 is electrically connected tothe scan line 30205; one of a source electrode and a drain electrode ofthe transistor 30201 is electrically connected to the video signal line30204; and the other of the source electrode and the drain electrode ofthe transistor 30201 is electrically connected to one of electrodes ofthe auxiliary capacitor 30202 and one of electrodes of the displayelement 30203. In addition, the other of the electrodes of the auxiliarycapacitor 30202 is electrically connected to the common line 30206.

First, in each of pixels selected by the scan line 30205, voltagecorresponding to a video signal is applied to the display element 30203and the auxiliary capacitor 30202 through the video signal line 30204because the transistor 30201 is turned on. At this time, when the videosignal is a signal which makes all pixels connected to the common line30206 display the lowest grayscale or when the video signal is a signalwhich makes all of the pixels connected to the common line 30206 displaythe highest grayscale, it is not necessary that the video signal bewritten to each pixel through the video signal line 30204. Instead ofwriting the video signal through the video signal line 30204, voltageapplied to the display element 30203 can be changed by changing apotential of the common line 30206.

Next, FIG. 89B illustrates a plurality of pixel circuits in which twocommon lines are provided with respect to one scan line in a displaydevice using a display element which has capacitive properties like aliquid crystal element. Each of the pixel circuits illustrated in FIG.89B includes a transistor 30211, an auxiliary capacitor 30212, a displayelement 30213, a video signal line 30214, a scan line 30215, a firstcommon line 30216, and a second common line 30217.

A gate electrode of the transistor 30211 is electrically connected tothe scan line 30215; one of a source electrode and a drain electrode ofthe transistor 30211 is electrically connected to the video signal line30214; and the other of the source electrode and the drain electrode ofthe transistor 30211 is electrically connected to one of electrodes ofthe auxiliary capacitor 30212 and one of electrodes of the displayelement 30213. In addition, the other of the electrodes of the auxiliarycapacitor 30212 is electrically connected to the first common line30216. Further, in a pixel which is adjacent to the pixel, the other ofthe electrodes of the auxiliary capacitor 30212 is electricallyconnected to the second common line 30217.

In the display device shown in FIG. 89B, the number of pixel circuitswhich are electrically connected to one common line is smaller.Therefore, by changing a potential of the first common line 30216 or thesecond common line 30217 instead of writing a video signal through thevideo signal line 30214, frequency of changing voltage applied to thedisplay element 30213 is significantly increased. In addition, sourceinversion driving or dot inversion driving can be performed. Byperforming source inversion driving or dot inversion driving,reliability of the element can be improved and a flicker can besuppressed.

Next, a scanning backlight is described with reference to FIGS. 90A to90C. FIG. 90A is a view showing a scanning backlight in which coldcathode tubes are apposed. The scanning backlight shown in FIG. 90Aincludes a diffusion plate 30301 and N pieces of cold cathode tubes30302-1 to 30302-N. The N pieces of the cold cathode tubes 30302-1 to30302-N are apposed behind the diffusion plate 30301, so that the Npieces of the cold cathode tubes 30302-1 to 30302-N can be scanned whileluminances thereof are changed.

Change in luminance of each of the cold cathode tubes in scanning isdescribed with reference to FIG. 90C. First, luminance of the coldcathode tube 30302-1 is changed for a certain period. After that,luminance of the cold cathode tube 30302-2 which is provided adjacent tothe cold cathode tube 30302-1 is changed for the same length of period.In this manner, luminance is changed sequentially from the cold cathodetube 30302-1 to the cold cathode tube 30302-N. Although luminance whichis changed for a certain period is set to be lower than originalluminance in FIG. 90C, it may also be higher than original luminance. Inaddition, although scanning is performed from the cold cathode tube30302-1 to the cold cathode tube 30302-N, scanning may alternatively beperformed from the cold cathode tube 30302-N to the cold cathode tube30302-1, which is in a reversed order.

By performing driving as in FIG. 90C, average luminance of the backlightcan be decreased. Therefore, power consumption of the backlight, whichmakes up a major part of power consumption of the liquid crystal displaydevice, can be reduced.

Note that an LED may be used as a light source of the scanningbacklight. The scanning backlight in that case is as shown in FIG. 90B.The scanning backlight shown in FIG. 90B includes a diffusion plate30311 and light sources 30312-1 to 30312-N in each of which LEDs areapposed. When an LED is used as the light source of the scanningbacklight, there is an advantage in that the backlight can be thin andlightweight. In addition, there is another advantage in that a colorreproduction range can be widened. Further, since the LEDs which areapposed in each of the light sources 30312-1 to 30312-N can be similarlyscanned, a dot-scanning backlight can also be obtained. By using thedot-scanning backlight, image quality of a moving image can be furtherimproved.

Note that when the LED is used as the light source of the backlight,driving can be performed by changing luminance as shown in FIG. 90C.

Next, high frequency driving is described with reference to FIGS. 91Aand 91B. FIG. 91A is a view in which one image and one intermediateimage are displayed in one frame period 30600. Reference numeral 30601denotes an image of the frame; 30602 denotes an intermediate image ofthe frame; 30603 denotes an image of the next frame; and 30604 denotesan intermediate image of the next frame.

The intermediate image 30602 of the frame may be an image which is madebased on video signals of the frame and the next frame. Alternatively,the intermediate image 30602 of the frame may be an image which is madefrom the image 30601 of the frame. Further alternatively, theintermediate image 30602 of the frame may be a black image. Thus, imagequality of a moving image of a hold-type display device can be improved.When one image and one intermediate image are displayed in the one frameperiod 30600, there is an advantage in that consistency with a framerate of the video signal can be easily obtained and an image processingcircuit is not complicated.

FIG. 91B is a view in which one image and two intermediate images aredisplayed in a period with two successive one frame periods 30600 (i.e.,two frame periods). Reference numeral 30611 denotes an image of theframe; 30612 denotes an intermediate image of the frame; 30613 denotesan intermediate image of the next frame; and 30614 denotes an image of aframe after next.

Each of the intermediate image 30612 of the frame and the intermediateimage 30613 of the next frame may be an image which is made based onvideo signals of the frame, the next frame, and the frame after next.Alternatively, each of the intermediate image 30612 of the frame and theintermediate image 30613 of the next frame may be a black image. Whenone image and two intermediate images are displayed in the two frameperiods, there is an advantage in that operating frequency of aperipheral driver circuit does not need to be so high to effectivelyimprove image quality of a moving image.

Although this embodiment mode has been described with reference tovarious drawings, the contents (Or part of the contents) described ineach drawing can be freely applied to, combined with, or replaced withthe contents (or part of the contents) described in another drawing.Further, much more drawings can be formed by combining each part in theabove-described drawings with another part.

Similarly, the contents (or part of the contents) described in eachdrawing in this embodiment mode can be freely applied to, combined with,or replaced with the contents (or part of the contents) described in adrawing in another embodiment mode. Further, much more drawings can beformed by combining each part in the drawings in this embodiment modewith part of another embodiment mode.

Note that this embodiment mode has described just examples of embodying,slightly transforming, modifying, improving, describing in detail, orapplying the contents (or part of the contents) described in otherembodiment modes, an example of related part thereof, or the like.Therefore, the contents described in other embodiment modes can befreely applied to, combined with, or replaced with the contentsdescribed in this embodiment mode.

Embodiment Mode 11

Embodiment Mode 11 will describe a peripheral portion of a liquidcrystal panel.

FIG. 55 illustrates an example of a liquid crystal display deviceincluding a so-called edge-light type backlight unit 20101 and a liquidcrystal panel 20107. An edge-light type corresponds to a type in which alight source is provided at an end portion of a backlight unit andfluorescence of the light source is emitted from the entirelight-emitting surface. The edge-light type backlight unit 20101 is thinand can save power.

The backlight unit 20101 includes a diffusion plate 20102, a light guideplate 20103, a reflection plate 20104, a lamp reflector 20105, and alight source 20106.

The light source 20106 has a function of emitting light as necessary.For example, as the light source 20106, a cold cathode tube, a hotcathode tube, a light-emitting diode, an inorganic EL element, anorganic EL element, or the like can be used.

FIGS. 56A to 56D each illustrate a detailed structure of the edge-lighttype backlight unit. Note that description of a diffusion plate, a lightguide plate, a reflection plate, and the like is omitted.

A backlight unit 20201 shown in FIG. 56A has a structure in which a coldcathode tube 20203 is used as a light source. In addition, a lampreflector 20202 is provided to efficiently reflect light from the coldcathode tube 20203. Such a structure is often used for a large-scaledisplay device because luminance from the cold cathode tube 20203 ishigh.

A backlight unit 20211 shown in FIG. 56B has a structure in whichlight-emitting diodes (LEDs) 20213 are used as light sources. Forexample, the light-emitting diodes (LEDs) 20213 which emit white lightare provided with a predetermined interval therebetween. In addition, alamp reflector 20212 is provided to efficiently reflect light from thelight-emitting diodes (LEDs) 20213.

A backlight unit 20221 shown in FIG. 56C has a structure in whichlight-emitting diodes (LEDs) 20223, light-emitting diodes (LEDs) 20224,and light-emitting diodes (LEDs) 20225 of R, G, and B are used as lightsources. The light-emitting diodes (LEDs) 20223, the light-emittingdiodes (LEDs) 20224, and the light-emitting diodes (LEDs) 20225 of R, Qand B are each provided with a predetermined interval therebetween. Byusing the light-emitting diodes (LEDs) 20223, the light-emitting diodes(LEDs) 20224, and the light-emitting diodes (LEDs) 20225 of R, G and B,color reproductivity can be improved. In addition, a lamp reflector20222 is provided to efficiently reflect light from the light-emittingdiodes.

A backlight unit 20231 shown in FIG. 56D has a structure in whichlight-emitting diodes (LEDs) 20233, light-emitting diodes (LEDs) 20234,and light-emitting diodes (LEDs) 20235 of R, G, and B are used as lightsources. For example, among the light-emitting diodes (LEDs) 20233, thelight-emitting diodes (LEDs) 20234, and the light-emitting diodes (LEDs)20235 of R, Q and B, the light-emitting diodes of a color with lowemission intensity (e.g., green) are provided more than otherlight-emitting diodes. By using the light-emitting diodes (LEDs) 20233,the light-emitting diodes (LEDs) 20234, and the light-emitting diodes(LEDs) 20235 of R, G, and B, color reproductivity can be improved. Inaddition, a lamp reflector 20232 is provided to efficiently reflectlight from the light-emitting diodes.

FIG. 59 illustrates an example of a liquid crystal display deviceincluding a so-called direct-type backlight unit and a liquid crystalpanel. A direct-type corresponds to a type in which a light source isprovided directly below a light-emitting surface and fluorescence of thelight source is emitted from the entire light-emitting surface. Thedirect-type backlight unit can efficiently utilize emitted lightquantity.

A backlight unit 20500 includes a diffusion plate 20501, alight-shielding plate 20502, a lamp reflector 20503, a light source20504, and a liquid crystal panel 20505.

The light source 20504 has a function of emitting light as necessary.For example, as the light source 20504, a cold cathode tube, a hotcathode tube, a light-emitting diode, an inorganic EL element, anorganic EL element, or the like can be used.

FIG. 57 is a view showing an example of a structure of a polarizingplate (also referred to as a polarizing film).

A polarizing film 20300 includes a protective film 20301, a substratefilm 20302, a PVA polarizing film 20303, a substrate film 20304, anadhesive layer 20305, and a release film 20306.

When the PVA polarizing film 20303 is sandwiched between films servingas substrates (the substrate film 20302 and the substrate film 20304),reliability can be improved. Note that the PVA polarizing film 20303 maybe sandwiched between triacetylcellulose (TAC) films with hightransparency and high durability. Note also that each of the substratefilms and the TAC films function as protective films of a polarizerincluded in the PVA polarizing film 20303.

The adhesive layer 20305 which is to be attached to a glass substrate ofthe liquid crystal panel is attached to one of the substrate films (thesubstrate film 20304). Note that the adhesive layer 20305 is formed byapplying an adhesive to one of the substrate films (the substrate film20304). The adhesive layer 20305 is provided with the release film 20306(a separate film).

The other of the substrate films (the substrate film 20302) is providedwith the protective film 20301.

A hard coating scattering layer (an anti-glare layer) may be provided ona surface of the polarizing film 20300. Since the surface of the hardcoating scattering layer has minute unevenness formed by AG treatmentand has an anti-glare function which scatters external light, reflectionof external light in the liquid crystal panel and surface reflection canbe prevented.

Note also that a plurality of optical thin film layers having differentrefractive indexes may be layered on the surface of the polarizing film20300 (also referred to as anti-reflection treatment or AR treatment).The plurality of layered optical thin film layers having differentrefractive indexes can reduce reflectivity on the surface by aninterference effect of light.

FIGS. 58A to 58C are diagrams each illustrating an example of a systemblock of the liquid crystal display device.

In a pixel portion 20405, signal lines 20412 which are extended from asignal line driver circuit 20403 are provided. In the pixel portion20405, scan lines 20410 which are extended from a scan line drivercircuit 20404 are also provided. In addition, a plurality of pixels aredisposed in matrix at intersection portions of the signal lines 20412and the scan lines 20410. Note that each of the plurality of pixelsincludes a switching element. Therefore, voltage for controllinginclination of liquid crystal molecules can be individually input toeach of the plurality of pixels. A structure in which a switchingelement is provided at each intersection portion in this manner isreferred to as an active matrix type. Note also that the presentinvention is not limited to such an active matrix type and a structureof a passive matrix type may be used. Since the passive matrix type doesnot have a switching element in each pixel, a process is simple.

A driver circuit portion 20408 includes a control circuit 20402, thesignal line driver circuit 20403, and the scan line driver circuit20404. A video signal 20401 is input to the control circuit 20402. Thecontrol circuit 20402 controls the signal line driver circuit 20403 andthe scan line driver circuit 20404 in accordance with the video signal20401. Therefore, the control circuit 20402 inputs a control signal toeach of the signal line driver circuit 20403 and the scan line drivercircuit 20404. Then, the signal line driver circuit 20403 inputs a videosignal to each of the signal lines 20412 and the scan line drivercircuit 20404 inputs a scan signal to each of the scan lines 20410.Then, the switching element included in the pixel is selected inaccordance with the scan signal and the video signal is input to a pixelelectrode of the pixel.

Note that the control circuit 20402 also controls a power source 20407in accordance with the video signal 20401. The power source 20407includes a unit for supplying power to a lighting unit 20406. As thelighting unit 20406, an edge-light type backlight unit or a direct-typebacklight unit can be used. Note also that a front light may be used asthe lighting unit 20406. A front light corresponds to a plate-likelighting unit including a luminous body and a light guiding body, whichis attached to the front surface side of a pixel portion and illuminatesthe whole area. By using such a lighting unit, the pixel portion can beuniformly illuminated with low power consumption.

As shown in FIG. 58B, the scan line driver circuit 20404 includescircuits functioning as a shift register 20441, a level shifter 20442,and a buffer 20443. A signal such as a gate start pulse (GSP) or a gateclock signal (GCK) is input to the shift register 20441.

As shown in FIG. 58C, the signal line driver circuit 20403 includescircuits functioning as a shift register 20431, a first latch 20432, asecond latch 20433, a level shifter 20434, and a buffer 20435. Thecircuit functioning as the buffer 20435 corresponds to a circuit whichhas a function of amplifying a weak signal and includes an operationalamplifier or the like. A signal such as a start pulse (SSP) is input tothe level shifter 20434 and data (DATA) such as a video signal is inputto the first latch 20432. A latch (LAT) signal can be temporally held inthe second latch 20433 and is concurrently input to the pixel portion20405. This is referred to as line-sequential driving. Therefore, when apixel performs not line sequential driving but dot-sequential driving,the second latch can be omitted.

Note that in this embodiment mode, various types of liquid crystalpanels can be used. For example, a structure in which a liquid crystallayer is sealed between two substrates can be used as a liquid crystalpanel. A transistor, a capacitor, a pixel electrode, an alignment film,or the like is formed over one of the substrates. A polarizing plate, aretardation plate, or a prism sheet may be provided on the surfaceopposite to a top surface of the one of the substrates. A color filter,a black matrix, an opposite electrode, an alignment film, or the like isprovided on the other of the substrates. Note that a polarizing plate ora retardation plate may be provided on the surface opposite to a topsurface of the other of the substrates. Note also that the color filterand the black matrix may be formed on the top surface of the one of thesubstrates. Note also that three-dimensional display can be performed byproviding a slit (a grid) on the top surface side of the one of thesubstrates or the surface opposite to the top surface side of the one ofthe substrates.

Note also that each of the polarizing plate, the retardation plate, andthe prism sheet can be provided between the two substrates.Alternatively, each of the polarizing plate, the retardation plate, andthe prism sheet can be attached to or unified with one of the twosubstrates.

Although this embodiment mode has been described with reference tovarious drawings, the contents (or part of the contents) described ineach drawing can be freely applied to, combined with, or replaced withthe contents (or part of the contents) described in another drawing.Further, much more drawings can be formed by combining each part in theabove-described drawings with another part.

Similarly, the contents (or part of the contents) described in eachdrawing in this embodiment mode can be freely applied to, combined with,or replaced with the contents (or part of the contents) described in adrawing in another embodiment mode. Further, much more drawings can beformed by combining each part in the drawings in this embodiment modewith part of another embodiment mode.

Note that this embodiment mode has described just examples of embodying,slightly transforming, modifying, improving, describing in detail, orapplying the contents (or part of the contents) described in otherembodiment modes, an example of related part thereof, or the like.Therefore, the contents described in other embodiment modes can befreely applied to, combined with, or replaced with the contentsdescribed in this embodiment mode.

Embodiment Mode 12

Embodiment Mode 12 will describe a pixel structure and an operation of apixel which can be applied to a liquid crystal display device.

Note that in this embodiment mode, as an operation mode of a liquidcrystal element, a TN (Twisted Nematic) mode, an IPS(In-Plane-Switching) mode, an FFS (Fringe Field Switching) mode, an MVA(Multi-domain Vertical Alignment) mode, a PVA (Patterned VerticalAlignment) mode, an ASM (Axially Symmetric aligned Micro-cell) mode, anOCB (Optical Compensated Birefringence) mode, an FLC (FerroelectricLiquid Crystal) mode, an AFLC (AntiFerroelectric Liquid Crystal) mode,or the like can be used.

FIG. 60A is a diagram showing an example of a pixel structure which canbe applied to the liquid crystal display device.

A pixel 40100 includes a transistor 40101, a liquid crystal element40102, and a capacitor 40103. A gate of the transistor 40101 isconnected to a wiring 40105. A first terminal of the transistor 40101 isconnected to a wiring 40104. A second terminal of the transistor 40101is connected to a first electrode of the liquid crystal element 40102and a first electrode of the capacitor 40103. A second electrode of theliquid crystal element 40102 corresponds to an opposite electrode 40107.A second electrode of the capacitor 40103 is connected to a wiring40106.

The wiring 40104 functions as a signal line. The wiring 40105 functionsas a scan line. The wiring 40106 functions as a capacitor line. Thetransistor 40101 functions as a switch. The capacitor 40103 functions asa storage capacitor.

The transistor 40101 may function as a switch, and the transistor 40101may be a p-channel transistor or an n-channel transistor.

FIG. 60B illustrates an example of a pixel structure which can beapplied to the liquid crystal display device. In particular, FIG. 60B isa diagram showing an example of a pixel structure which can be appliedto a liquid crystal display device suitable for a lateral electricfield-mode (including an IPS-mode and an FFS-mode).

A pixel 40110 includes a transistor 40111, a liquid crystal element40112, and a capacitor 40113. A gate of the transistor 40111 isconnected to a wiring 40115. A first terminal of the transistor 40111 isconnected to a wiring 40114. A second terminal of the transistor 40111is connected to a first electrode of the liquid crystal element 40112and a first electrode of the capacitor 40113. A second electrode of theliquid crystal element 40112 is connected to a wiring 40116. A secondelectrode of the capacitor 40113 is connected to the wiring 40116.

The wiring 40114 functions as a signal line. The wiring 40115 functionsas a scan line. The wiring 40116 functions as a capacitor line. Thetransistor 40111 functions as a switch. The capacitor 40113 functions asa storage capacitor.

The transistor 40111 may function as a switch, and the transistor 40111may be a p-channel transistor or an n-channel transistor.

FIG. 61 illustrates an example of a pixel structure which can be appliedto the liquid crystal display device. In particular, FIG. 61 illustratesan example of a pixel structure with which an aperture ratio of a pixelcan be increased by reducing the number of wirings.

FIG. 61 illustrates two pixels which are provided in the same columndirection (a pixel 40200 and a pixel 40210). For example, when the pixel40200 is provided at the N-th row, the pixel 40210 is provided at the(N+1)th row.

The pixel 40200 includes a transistor 40201, a liquid crystal element40202, and a capacitor 40203. A gate of the transistor 40201 isconnected to a wiring 40205. A first terminal of the transistor 40201 isconnected to a wiring 40204. A second terminal of the transistor 40201is connected to a first electrode of the liquid crystal element 40202and a first electrode of the capacitor 40203. A second electrode of theliquid crystal element 40202 corresponds to an opposite electrode 40207.A second electrode of the capacitor 40203 is connected to the samewiring as a gate of a transistor of the previous row.

The pixel 40210 includes a transistor 40211, a liquid crystal element40212, and a capacitor 40213. A gate of the transistor 40211 isconnected to a wiring 40215. A first terminal of the transistor 40211 isconnected to the wiring 40204. A second terminal of the transistor 40211is connected to a first electrode of the liquid crystal element 40212and a first electrode of the capacitor 40213. A second electrode of theliquid crystal element 40212 corresponds to an opposite electrode 40217.A second electrode of the capacitor 40213 is connected to the samewiring (the wiring 40205) as the gate of the transistor of the previousrow.

The wiring 40204 functions as a signal line. The wiring 40205 functionsas a scan line of the N-th row. The wiring 40205 also functions as acapacitor line of the (N+1)th row. The transistor 40201 functions as aswitch. The capacitor 40203 functions as a storage capacitor.

The wiring 40215 functions as a scan line of the (N+1)th row. The wiring40215 also functions as a capacitor line of an (N+2)th row. Thetransistor 40211 functions as a switch. The capacitor 40213 functions asa storage capacitor.

Each of the transistor 40201 and the transistor 40211 may function as aswitch, and each of the transistor 40201 and the transistor 40211 may bea p-channel transistor or an n-channel transistor.

FIG. 62 illustrates an example of a pixel structure which can be appliedto the liquid crystal display device. In particular, FIG. 62 illustratesan example of a pixel structure with which a viewing angle can beimproved by using a subpixel.

A pixel 40320 includes a subpixel 50300 and a subpixel 40310. Although acase where the pixel 40320 includes two subpixels is described, thepixel 40320 may include three or more subpixels.

The subpixel 40300 includes a transistor 40301, a liquid crystal element40302, and a capacitor 40303. A gate of the transistor 40301 isconnected to a wiring 40305. A first terminal of the transistor 40301 isconnected to a wiring 40304. A second terminal of the transistor 40301is connected to a first electrode of the liquid crystal element 40302and a first electrode of the capacitor 40303. A second electrode of theliquid crystal element 40302 corresponds to an opposite electrode 40307.A second electrode of the capacitor 40303 is connected to a wiring40306.

The subpixel 40310 includes a transistor 40311, a liquid crystal element40312, and a capacitor 40313. A gate of the transistor 40311 isconnected to a wiring 40315. A first terminal of the transistor 40311 isconnected to the wiring 40304. A second terminal of the transistor 40311is connected to a first electrode of the liquid crystal element 40312and a first electrode of the capacitor 40313. A second electrode of theliquid crystal element 40312 corresponds to an opposite electrode 40317.A second electrode of the capacitor 40313 is connected to the wiring40306.

The wiring 40304 functions as a signal line. The wiring 40305 functionsas a scan line. The wiring 40315 functions as a signal line. The wiring40306 functions as a capacitor line. The transistor 40301 functions as aswitch. The transistor 40311 functions as a switch. The capacitor 40303functions as a storage capacitor. The capacitor 40313 functions as astorage capacitor.

The transistor 40301 may function as a switch, and the transistor 40301may be a p-channel transistor or an n-channel transistor. The transistor40311 may function as a switch, and the transistor 40311 may be ap-channel transistor or an n-channel transistor.

A video signal input to the subpixel 40300 may be a value which isdifferent from that of a video signal input to the subpixel 40310. Inthis case, the viewing angle can be widened because alignment of liquidcrystal molecules of the liquid crystal element 40302 and alignment ofliquid crystal molecules of the liquid crystal element 40312 can bevaried from each other.

Although this embodiment mode has been described with reference tovarious drawings, the contents (or part of the contents) described ineach drawing can be freely applied to, combined with, or replaced withthe contents (or part of the contents) described in another drawing.Further, much more drawings can be formed by combining each part in theabove-described drawings with another part.

Similarly, the contents (or part of the contents) described in eachdrawing in this embodiment mode can be freely applied to, combined with,or replaced with the contents (or part of the contents) described in adrawing in another embodiment mode. Further, much more drawings can beformed by combining each part in the drawings in this embodiment modewith part of another embodiment mode.

Note that this embodiment mode has described just examples of embodying,slightly transforming, modifying, improving, describing in detail, orapplying the contents (or part of the contents) described in otherembodiment modes, an example of related part thereof, or the like.Therefore, the contents described in other embodiment modes can befreely applied to, combined with, or replaced with the contentsdescribed in this embodiment mode.

Embodiment Mode 13

This embodiment mode describes various liquid crystal modes.

First, various liquid crystal modes are described with reference tocross-sectional views.

FIGS. 63A and 63B are schematic views of cross sections of a TN mode.

A liquid crystal layer 50100 is sandwiched between a first substrate50101 and a second substrate 50102 which are arranged to be opposite toeach other. A first electrode 50105 is formed on a top surface of thefirst substrate 50101. A second electrode 50106 is formed on a topsurface of the second substrate 50102. A first polarizing plate 50103 isprovided on the first substrate 50101 on a side opposite to the liquidcrystal layer 50100. A second polarizing plate 50104 is provided on thesecond substrate 50102 on a side opposite to the liquid crystal layer50100. Note that the first polarizing plate 50103 and the secondpolarizing plate 50104 are arranged so as to be in a cross nicol state.

The first polarizing plate 50103 may be provided on the top surface ofthe first substrate 50101, that is, between the first substrate 50101and the liquid crystal layer 50100. The second polarizing plate 50104may be provided on the top surface of the second substrate 50102, thatis, between the second substrate 50102 and the liquid crystal layer50100.

It is only necessary that at least one of the first electrode 50105 andthe second electrode 50106 have light-transmitting properties (atransmissive or reflective liquid crystal display device).Alternatively, both the first electrode 50105 and the second electrode50106 may have light-transmitting properties, and part of one of theelectrodes may have reflectivity (a transflective liquid crystal displaydevice).

FIG. 63A is a schematic view of a cross section in the case wherevoltage is applied to the first electrode 50105 and the second electrode50106 (referred to as a vertical electric field mode).

FIG. 63B is a schematic view of a cross section in the case wherevoltage is not applied to the first electrode 50105 and the secondelectrode 50106.

FIGS. 64A and 64B are schematic views of cross sections of a VA mode. Inthe VA mode, liquid crystal molecules are aligned such that they arevertical to a substrate when there is no electric field.

A liquid crystal layer 50200 is sandwiched between a first substrate50201 and a second substrate 50202 which are arranged to be opposite toeach other. A first electrode 50205 is formed on a top surface of thefirst substrate 50201. A second electrode 50206 is formed on a topsurface of the second substrate 50202. A first polarizing plate 50203 isprovided on the first substrate 50201 on a side opposite to the liquidcrystal layer 50200. A second polarizing plate 50204 is provided on thesecond substrate 50202 on a side opposite to the liquid crystal layer50200. Note that the first polarizing plate 50203 and the secondpolarizing plate 50204 are arranged so as to be in a cross nicol state.

The first polarizing plate 50203 may be provided on the top surface ofthe first substrate 50201, that is, between the first substrate 50201and the liquid crystal layer 50200. The second polarizing plate 50204may be provided on the top surface of the second substrate 50202, thatis, between the second substrate 50202 and the liquid crystal layer50200.

It is only necessary that at least one of the first electrode 50205 andthe second electrode 50206 have light-transmitting properties (atransmissive or reflective liquid crystal display device).Alternatively, both the first electrode 50205 and the second electrode50206 may have light-transmitting properties, and part of one of theelectrodes may have reflectivity (a transflective liquid crystal displaydevice).

FIG. 64A is a schematic view of a cross section in the case wherevoltage is applied to the first electrode 50205 and the second electrode50206 (referred to as a vertical electric field mode).

FIG. 64B is a schematic view of a cross section in the case wherevoltage is not applied to the first electrode 50205 and the secondelectrode 50206.

FIGS. 64C and 64D are schematic views of cross sections of an MVA mode.In the MVA mode, viewing angle dependency of each portion is compensatedby each other.

A liquid crystal layer 50210 is sandwiched between a first substrate50211 and a second substrate 50212 which are arranged to be opposite toeach other. A first electrode 50215 is formed on a top surface of thefirst substrate 50211. A second electrode 50216 is formed on a topsurface of the second substrate 50212. A first projection 50217 forcontrolling alignment is formed on the first electrode 50215. A secondprojection 50218 for controlling alignment is formed over the secondelectrode 50216. A first polarizing plate 50213 is provided on the firstsubstrate 50211 on a side opposite to the liquid crystal layer 50210. Asecond polarizing plate 50214 is provided on the second substrate 50212on a side opposite to the liquid crystal layer 50210. Note that thefirst polarizing plate 50213 and the second polarizing plate 50214 arearranged so as to be in a cross nicol state.

The first polarizing plate 50213 may be provided on the top surface ofthe first substrate 50211, that is, between the first substrate 50211and the liquid crystal layer 50210. The second polarizing plate 50214may be provided on the top surface of the second substrate 50212, thatis, between the second substrate 50212 and the liquid crystal layer50210.

It is only necessary that at least one of the first electrode 50215 andthe second electrode 50216 have light-transmitting properties (atransmissive or reflective liquid crystal display device).Alternatively, both the first electrode 50215 and the second electrode50216 may have light-transmitting properties, and part of one of theelectrodes may have reflectivity (a transflective liquid crystal displaydevice).

FIG. 64C is a schematic view of a cross section in the case wherevoltage is applied to the first electrode 50215 and the second electrode50216 (referred to as a vertical electric field mode).

FIG. 64D is a schematic view of a cross section in the case wherevoltage is not applied to the first electrode 50215 and the secondelectrode 50216.

FIGS. 65A and 65B are schematic views of cross sections of an OCB mode.In the OCB mode, viewing angle dependency is low because alignment ofliquid crystal molecules in a liquid crystal layer can be opticallycompensated. This state of the liquid crystal molecules is referred toas bend alignment.

A liquid crystal layer 50300 is sandwiched between a first substrate50301 and a second substrate 50302 which are arranged to be opposite toeach other. A first electrode 50305 is formed on a top surface of thefirst substrate 50301. A second electrode 50306 is formed on a topsurface of the second substrate 50302. A first polarizing plate 50303 isprovided on the first substrate 50301 on a side opposite to the liquidcrystal layer 50300. A second polarizing plate 50304 is provided on thesecond substrate 50302 on a side opposite to the liquid crystal layer50300. Note that the first polarizing plate 50303 and the secondpolarizing plate 50304 are arranged so as to be in a cross nicol state.

The first polarizing plate 50303 may be provided on the top surface ofthe first substrate 50301, that is, between the first substrate 50301and the liquid crystal layer 50300. The second polarizing plate 50304may be provided on the top surface of the second substrate 50302, thatis, between the second substrate 50302 and the liquid crystal layer50300.

It is only necessary that at least one of the first electrode 50305 andthe second electrode 50306 have light-transmitting properties (atransmissive or reflective liquid crystal display device).Alternatively, both the first electrode 50305 and the second electrode50306 may have light-transmitting properties, and part of one of theelectrodes may have reflectivity (a transflective liquid crystal displaydevice).

FIG. 65A is a schematic view of a cross section in the case wherevoltage is applied to the first electrode 50305 and the second electrode50306 (referred to as a vertical electric field mode).

FIG. 65B is a schematic view of a cross section in the case wherevoltage is not applied to the first electrode 50305 and the secondelectrode 50306.

FIGS. 65C and 65D are schematic views of cross sections of an FLC modeor an AFLC mode.

A liquid crystal layer 50310 is sandwiched between a first substrate50311 and a second substrate 50312 which are arranged to be opposite toeach other. A first electrode 50315 is formed on a top surface of thefirst substrate 50311. A second electrode 50316 is formed on a topsurface of the second substrate 50312. A first polarizing plate 50313 isprovided on the first substrate 50311 on a side opposite to the liquidcrystal layer 50310. A second polarizing plate 50314 is provided on thesecond substrate 50312 on a side opposite to the liquid crystal layer50310. Note that the first polarizing plate 50313 and the secondpolarizing plate 50314 are arranged so as to be in a cross nicol state.

The first polarizing plate 50313 may be provided on the top surface ofthe first substrate 50311, that is, between the first substrate 50311and the liquid crystal layer 50310. The second polarizing plate 50314may be provided on the top surface of the second substrate 50312, thatis, between the second substrate 50312 and the liquid crystal layer50310.

It is only necessary that at least one of the first electrode 50315 andthe second electrode 50316 have light-transmitting properties (atransmissive or reflective liquid crystal display device).Alternatively, both the first electrode 50315 and the second electrode50316 may have light-transmitting properties, and part of one of theelectrodes may have reflectivity (a transflective liquid crystal displaydevice).

FIG. 65C is a schematic view of a cross section in the case wherevoltage is applied to the first electrode 50315 and the second electrode50316 (referred to as a vertical electric field mode).

FIG. 65D is a schematic view of a cross section in the case wherevoltage is not applied to the first electrode 50315 and the secondelectrode 50316.

FIGS. 66A and 66B are schematic views of cross sections of an IPS mode.In the IPS mode, alignment of liquid crystal molecules in a liquidcrystal layer can be optically compensated, the liquid crystal moleculesare constantly rotated in a plane parallel to a substrate, and ahorizontal electric field method in which electrodes are provided onlyon one substrate is used.

A liquid crystal layer 50400 is sandwiched between a first substrate50401 and a second substrate 50402 which are arranged to be opposite toeach other. A first electrode 50405 and a second electrode 50406 areformed on a top surface of the second substrate 50402. A firstpolarizing plate 50403 is provided on the first substrate 50401 on aside opposite to the liquid crystal layer 50400. A second polarizingplate 50404 is provided on a surface of the second substrate 50402,which does not face the liquid crystal layer 50400. Note that the firstpolarizing plate 50403 and the second polarizing plate 50404 arearranged so as to be in a cross nicol state.

The first polarizing plate 50403 may be provided on the top surface ofthe first substrate 50401, that is, between the first substrate 50401and the liquid crystal layer 50400. The second polarizing plate 50404may be provided on the top surface of the second substrate 50402, thatis, may be provided between the second substrate 50402 and the liquidcrystal layer 50400.

It is only necessary that at least one of the first electrode 50405 andthe second electrode 50406 have light-transmitting properties (atransmissive or reflective liquid crystal display device).Alternatively, both the first electrode 50405 and the second electrode50406 may have light-transmitting properties, and part of one of theelectrodes may have reflectivity (a transflective liquid crystal displaydevice).

FIG. 66A is a schematic view of a cross section in the case wherevoltage is applied to the first electrode 50405 and the second electrode50406 (referred to as a vertical electric field mode).

FIG. 66B is a schematic view of a cross section in the case wherevoltage is not applied to the first electrode 50405 and the secondelectrode 50406.

FIGS. 66C and 66D are schematic views of cross sections of an FFS mode.In the FFS mode, alignment of liquid crystal molecules in a liquidcrystal layer can be optically compensated, the liquid crystal moleculesare constantly rotated in a plane parallel to a substrate, and ahorizontal electric field method in which electrodes are provided onlyon one substrate is used.

A liquid crystal layer 50410 is sandwiched between a first substrate50411 and a second substrate 50412 which are arranged to be opposite toeach other. A second electrode 50416 is formed on a top surface of thesecond substrate 50412. An insulating film 50417 is formed on a topsurface of the second electrode 50416. A first electrode 50415 is formedover the insulating film 50417. A first polarizing plate 50413 isprovided on the first substrate 50411 on a side opposite to the liquidcrystal layer 50410. A second polarizing plate 50414 is provided on thesecond substrate 50412 on a side opposite to the liquid crystal layer50410. Note that the first polarizing plate 50413 and the secondpolarizing plate 50414 are arranged so as to be in a cross nicol state.

The first polarizing plate 50413 may be provided on the top surface ofthe first substrate 50411, that is, between the first substrate 50411and the liquid crystal layer 50410. The second polarizing plate 50414may be provided on the top surface of the second substrate 50412, thatis, may be provided between the second substrate 50412 and the liquidcrystal layer 50410.

It is only necessary that at least one of the first electrode 50415 andthe second electrode 50416 have light-transmitting properties (atransmissive or reflective liquid crystal display device).Alternatively, both the first electrode 50415 and the second electrode50416 may have light-transmitting properties, and part of one of theelectrodes may have reflectivity (a transflective liquid crystal displaydevice).

FIG. 66C is a schematic view of a cross section in the case wherevoltage is applied to the first electrode 50415 and the second electrode50416 (referred to as a vertical electric field mode).

FIG. 66D is a schematic view of a cross section in the case wherevoltage is not applied to the first electrode 50415 and the secondelectrode 50416.

Although this embodiment mode has been described with reference tovarious drawings, the contents (or part of the contents) described ineach drawing can be freely applied to, combined with, or replaced withthe contents (or part of the contents) described in another drawing.Further, much more drawings can be formed by combining each part in theabove-described drawings with another part.

Similarly, the contents (or part of the contents) described in eachdrawing in this embodiment mode can be freely applied to, combined with,or replaced with the contents (or part of the contents) described in adrawing in another embodiment mode. Further, much more drawings can beformed by combining each part in the drawings in this embodiment modewith part of another embodiment mode.

Note that this embodiment mode has described just examples of embodying,slightly transforming, modifying, improving, describing in detail, orapplying the contents (or part of the contents) described in otherembodiment modes, an example of related part thereof, or the like.Therefore, the contents described in other embodiment modes can befreely applied to, combined with, or replaced with the contentsdescribed in this embodiment mode.

Embodiment Mode 14

This embodiment mode describes a pixel structure of a display device. Inparticular, it describes a pixel structure of a liquid crystal displaydevice.

Pixel structures in the case where each liquid crystal mode and atransistor are combined are described with reference to cross-sectionalviews of pixels.

As the transistor, a thin film transistor (TFT) including a non-singlecrystalline semiconductor layer typified by amorphous silicon,polycrystalline silicon, microcrystalline (also referred to assemi-amorphous) silicon, or the like can be used.

The transistor can have a top-gate structure, a bottom-gate structure,or the like. Note that the bottom-gate transistor can be achannel-etched transistor, a channel-protective transistor, or the like.

FIG. 67 shows an example of a cross-sectional view of a pixel in thecase where a TN mode and a transistor are combined. A liquid crystal10111 having liquid crystal molecules 10118 is sandwiched between afirst substrate 10101 and a second substrate 10116. The first substrate10101 is provided with a transistor, a pixel electrode, an alignmentfilm, and the like. The second substrate 10116 is provided with alight-blocking film 10114, a color filter 10115, an opposite electrode,an alignment film, and the like. In addition, a spacer 10117 is providedbetween the first substrate 10101 and the second substrate 10116. Byapplying the pixel structure shown in FIG. 67 to a liquid crystaldisplay device, a liquid crystal display device can be formed at lowcost.

FIG. 68A shows an example of a cross-sectional view of a pixel in thecase where an MVA (Multi-domain Vertical Alignment) mode and atransistor are combined. A liquid crystal 10211 having liquid crystalmolecules 10218 is sandwiched between a first substrate 10201 and asecond substrate 10216. The first substrate 10201 is provided with atransistor, a pixel electrode, an alignment film, and the like. Thesecond substrate 10216 is provided with a light-blocking film 10214, acolor filter 10215, an opposite electrode, a projection 10219 foralignment control, an alignment film, and the like. In addition, aspacer 10217 is provided between the first substrate 10201 and thesecond substrate 10216. By applying the pixel structure shown in FIG.68A to a liquid crystal display device, a liquid crystal display devicehaving a wide viewing angle, high response speed, and high contrast canbe obtained.

FIG. 68B shows an example of a cross-sectional view of a pixel in thecase where a PVA (Patterned Vertical Alignment) mode and a transistorare combined. A liquid crystal 10241 having liquid crystal molecules10248 is sandwiched between a first substrate 10231 and a secondsubstrate 10246. The first substrate 10231 is provided with atransistor, a pixel electrode, an alignment film, and the like. Thesecond substrate 10246 is provided with a light-blocking film 10244, acolor filter 10245, an opposite electrode, an alignment film, and thelike. Note that the pixel electrode includes an electrode notch portion10249. In addition, a spacer 10247 is provided between the firstsubstrate 10231 and the second substrate 10246. By applying the pixelstructure shown in FIG. 68B to a liquid crystal display device, a liquidcrystal display device having a wide viewing angle, high response speed,and high contrast can be obtained.

FIG. 69A shows an example of a cross-sectional view of a pixel in thecase where an IPS (In-Plane-Switching) mode and a transistor arecombined. A liquid crystal 10311 having liquid crystal molecules 10318is sandwiched between a first substrate 10301 and a second substrate10316. The first substrate 10301 is provided with a transistor, a pixelelectrode, a common electrode, an alignment film, and the like. Thesecond substrate 10316 is provided with a light-blocking film 10314, acolor filter 10315, an alignment film, and the like. In addition, aspacer 10317 is provided between the first substrate 10301 and thesecond substrate 10316. By applying the pixel structure shown in FIG.69A to a liquid crystal display device, a liquid crystal display devicehaving a wide viewing angle and response speed with low dependency ongray scale in principle can be obtained.

FIG. 69B shows an example of a cross-sectional view of a pixel in thecase where an FFS (Fringe Field Switching) mode and a transistor arecombined. A liquid crystal 10341 having liquid crystal molecules 10348is sandwiched between a first substrate 10331 and a second substrate10346. The first substrate 10331 is provided with a transistor, a pixelelectrode, a common electrode, an alignment film, and the like. Thesecond substrate 10346 is provided with a light-blocking film 10344, acolor filter 10345, an alignment film, and the like. In addition, aspacer 10347 is provided between the first substrate 10331 and thesecond substrate 10346. By applying the pixel structure shown in FIG.69B to a liquid crystal display device, a liquid crystal display devicehaving a wide viewing angle and response speed with low dependency ongray scale in principle can be obtained.

Here, materials which can be used for conductive layers or insulatingfilms are described.

As a first insulating film 10102 in FIG. 67, a first insulating film10202 in FIG. 68A, a first insulating film 10232 in FIG. 68B, a firstinsulating film 10302 in FIG. 69A, and a first insulating film 10332 inFIG. 69B, an insulating film such as a silicon oxide film, a siliconnitride film, or a silicon oxynitride (SiO_(x)N_(y)) film can be used.Alternatively, an insulating film having a stacked-layer structure inwhich two or more of a silicon oxide film, a silicon nitride film, asilicon oxynitride (SiO_(x)N_(y)) film, and the like are combined can beused.

As a first conductive layer 10103 in FIG. 67, a first conductive layer10203 in FIG. 68A, a first conductive layer 10233 in FIG. 68B, a firstconductive layer 10303 in FIG. 69A, and a first conductive layer 10333in FIG. 69B, Mo, Ti, Al, Nd, Cr, or the like can be used. Alternatively,a stacked-layer structure in which two or more of Mo, Ti, Al, Nd, Cr,and the like are combined can be used.

As a second insulating film 10104 in FIG. 67, a second insulating film10204 in FIG. 68A, a second insulating film 10234 in FIG. 68B, a secondinsulating film 10304 in FIG. 69A, and a second insulating film 10334 inFIG. 69B, a thermal oxide film, a silicon oxide film, a silicon nitridefilm, a silicon oxynitride film, or the like can be used. Alternatively,a stacked-layer structure in which two or more of a thermal oxide film,a silicon oxide film, a silicon nitride film, a silicon oxynitride film,and the like are combined can be used. Note that a silicon oxide film ispreferably used for a portion in contact with a semiconductor layer.This is because a trap level at an interface with the semiconductorlayer is decreased when a silicon oxide film is used. Note also that asilicon nitride film is preferably used for a portion in contact withMo. This is because a silicon nitride film does not oxidize Mo.

As a first semiconductor layer 10105 in FIG. 67, a first semiconductorlayer 10205 in FIG. 68A, a first semiconductor layer 10235 in FIG. 68B,a first semiconductor layer 10305 in FIG. 69A, and a first semiconductorlayer 10335 in FIG. 69B, silicon, silicon germanium (SiGe), or the likecan be used.

As a second semiconductor layer 10106 in FIG. 67, a second semiconductorlayer 10206 in FIG. 68A, a second semiconductor layer 10236 in FIG. 68B,a second semiconductor layer 10306 in FIG. 69A, and a secondsemiconductor layer 10336 in FIG. 69B, silicon including phosphorus orthe like can be used, for example.

As a light-transmitting material used for a second conductive layer10107, a third conductive layer 10109, and a fourth conductive layer10113 in FIG. 67; a second conductive layer 10207, a third conductivelayer 10209, and a fourth conductive layer 10213 in FIG. 68A; a secondconductive layer 10237, a third conductive layer 10239, and a fourthconductive layer 10243 in FIG. 68B; a second conductive layer 10307 anda third conductive layer 10309 in FIG. 69A; and a second conductivelayer 10337, a third conductive layer 10339, and a fourth conductivelayer 10343 in FIG. 69B, an indium tin oxide (ITO) film formed by mixingtin oxide into indium oxide, an indium tin silicon oxide (ITSO) filmformed by mixing silicon oxide into indium tin oxide (ITO), an indiumzinc oxide (IZO) film formed by mixing zinc oxide into indium oxide, azinc oxide film, a tin oxide film, or the like can be used. Note thatIZO is a light-transmitting conductive material formed by sputteringusing a target in which zinc oxide (ZnO) of 2 to 20 wt % is mixed intoITO.

As a reflective material used for the second conductive layer 10107 andthe third conductive layer 10109 in FIG. 67; the second conductive layer10207 and the third conductive layer 10209 in FIG. 68A; the secondconductive layer 10237 and the third conductive layer 10239 in FIG. 68B;the second conductive layer 10307 and the third conductive layer 10309in FIG. 69A; and the second conductive layer 10337, the third conductivelayer 10339, and the fourth conductive layer 10343 in FIG. 68B, Ti, Mo,Ta, Cr, W, Al, or the like can be used. Alternatively, a two-layerstructure in which Al and Ti, Mo, Ta, Cr, or W are stacked, or athree-layer structure in which Al is interposed between metals such asTi, Mo, Ta, Cr, and W may be used.

As the third insulating film 10108 in FIG. 67, the third insulating film10208 in FIG. 68A, the third insulating film 10238 in FIG. 68B, thethird conductive layer 10239 in FIG. 68B, the third insulating film10308 in FIG. 69A, and the third insulating film 10338 and the fourthinsulating film 10349 in FIG. 69B, an inorganic material (e.g., siliconoxide, silicon nitride, or silicon oxynitride), an organic compoundmaterial having a low dielectric constant (e.g., a photosensitive ornonphotosensitive organic resin material), or the like can be used.Alternatively, a material including siloxane can be used. Note thatsiloxane is a material in which a skeleton structure is formed by a bondof silicon (Si) and oxygen (O). As a substitute, an organic groupcontaining at least hydrogen (such as an alkyl group or an aryl group)is used. Alternatively, a fluoro group, or a fluoro group and an organicgroup containing at least hydrogen may be used as a substituent.

As a first alignment film 10110 and a second alignment film 10112 inFIG. 67; a first alignment film 10210 and a second alignment film 10212in FIG. 68A; a first alignment film 10240 and a second alignment film10242 in FIG. 68B; a first alignment film 10310 and a second alignmentfilm 10312 in FIG. 69A; and a first alignment film 10340 and a secondalignment film 10342 in FIG. 69B, a film of a high molecular compoundsuch as polyimide can be used.

Next, the pixel structure in the case where each liquid crystal mode andthe transistor are combined is described with reference to a top planview (a layout diagram) of the pixel.

Note that as the liquid crystal mode, a TN (Twisted Nematic) mode, anIPS (In-Plane-Switching) mode, an FFS (Fringe Field Switching) mode, anMVA (Multi-domain Vertical Alignment) mode, a PVA (Patterned VerticalAlignment) mode, an ASM (Axially Symmetric aligned Micro-cell) mode, anOCB (Optical Compensated Birefringence) mode, an FLC (FerroelectricLiquid Crystal) mode, an AFLC (AntiFerroelectric Liquid Crystal) mode,or the like can be used.

FIG. 70 shows an example of a top plan view of a pixel in the case wherea TN mode and a transistor are combined. By applying the pixel structureshown in FIG. 70 to a liquid crystal display device, a liquid crystaldisplay device can be formed at low cost.

The pixel shown in FIG. 70 includes a scan line 10401, a video signalline 10402, a capacitor line 10403, a transistor 10404, a pixelelectrode 10405, and a pixel capacitor 10406.

FIG. 71A shows an example of a top plan view of a pixel in the casewhere an MVA mode and a transistor are combined. By applying the pixelstructure shown in FIG. 71A to a liquid crystal display device, a liquidcrystal display device having a wide viewing angle, high response speed,and high contrast can be obtained.

The pixel shown in FIG. 71A includes a scan line 10501, a video signalline 10502, a capacitor line 10503, a transistor 10504, a pixelelectrode 10505, a pixel capacitor 10506, and a projection 10507 foralignment control.

FIG. 71B shows an example of a top plan view of a pixel in the casewhere a PVA mode and a transistor are combined. By applying the pixelstructure shown in FIG. 71B to a liquid crystal display device, a liquidcrystal display device having a wide viewing angle, high response speed,and high contrast can be obtained.

The pixel shown in FIG. 71B includes a scan line 10511, a video signalline 10512, a capacitor line 10513, a transistor 10514, a pixelelectrode 10515, a pixel capacitor 10516, and an electrode notch portion10517.

FIG. 72A shows an example of a top plan view of a pixel in the casewhere an IPS mode and a transistor are combined. By applying the pixelstructure shown in FIG. 72A to a liquid crystal display device, a liquidcrystal display device having a wide viewing angle and response speedwith low dependency on gray scale in principle can be obtained.

The pixel shown in FIG. 72A includes a scan line 10601, a video signalline 10602, a common electrode 10603, a transistor 10604, and a pixelelectrode 10605.

FIG. 72B shows an example of a top plan view of a pixel in the casewhere an FFS mode and a transistor are combined. By applying the pixelstructure shown in FIG. 72B to a liquid crystal display device, a liquidcrystal display device having a wide viewing angle and response speedwith low dependency on gray scale in principle can be obtained.

The pixel shown in FIG. 72B includes a scan line 10611, a video signalline 10612, a common electrode 10613, a transistor 10614, and a pixelelectrode 10615.

Although this embodiment mode has been described with reference tovarious drawings, the contents (or part of the contents) described ineach drawing can be freely applied to, combined with, or replaced withthe contents (or part of the contents) described in another drawing.Further, much more drawings can be formed by combining each part in theabove-described drawings with another part.

Similarly, the contents (or part of the contents) described in eachdrawing in this embodiment mode can be freely applied to, combined with,or replaced with the contents (or part of the contents) described in adrawing in another embodiment mode. Further, much more drawings can beformed by combining each part in the drawings in this embodiment modewith part of another embodiment mode.

Note that this embodiment mode has described just examples of embodying,slightly transforming, modifying, improving, describing in detail, orapplying the contents (or part of the contents) described in otherembodiment modes, an example of related part thereof, or the like.Therefore, the contents described in other embodiment modes can befreely applied to, combined with, or replaced with the contentsdescribed in this embodiment mode.

Embodiment Mode 15

Embodiment Mode 15 will describe a structure and an operation of a pixelin a display device.

FIGS. 73A and 73B are timing charts showing an example of digital timegrayscale driving. The timing chart of FIG. 73A illustrates a drivingmethod in which a signal writing period (address period) to a pixel anda light-emitting period (sustain period) are divided.

One frame period is a period for fully displaying an image of onedisplay region. One frame period includes a plurality of subframeperiods, and one subframe period includes an address period and asustain period. Address periods Ta1 to Ta4 indicate time for writingsignals to pixels of all rows, and periods Tb1 to Tb4 indicate time forwriting signals to pixels of one row (or one pixel). Sustain periods Ts1to Ts4 indicate time for maintaining a lighting state or a non-lightingstate in accordance with a video signal written to the pixel, and aratio of the lengths of the sustain periods is set to satisfyTs1:Ts2:Ts3:Ts4=2³:2²:2¹:2⁰=8:4:2:1. A grayscale is expressed dependingon which sustain period light emission is performed.

Here, the i-th pixel row is described with reference to FIG. 73B. First,in the address period Ta1, a pixel selection signal is input to a scanline in order from a first row, and in a period Tb1(i) in the addressperiod Ta1, pixels of the i-th row are selected. Then, while the pixelsof the i-th row are selected, a video signal is input to the pixels ofthe i-th row from a signal line. Then, when the video signal is writtento the pixels of the i-th row, the pixels of the i-th row maintain thesignal until a signal is input again. Lighting and non-lighting of thepixels of the i-th row in the sustain period Ts1 are controlled by thewritten video signal. Similarly, in the address periods Ta2, Ta3, andTa4, video signals are input to the pixels of the i-th row, and lightingand non-lighting of the pixels of the i-th row in the sustain periodsTs2, Ts3, and Ts4 are controlled by the video signals. Then, in eachsubframe period, pixels are not lit in the address period, and thesustain period starts after the address period ends, and pixels to whicha signal for lighting is written are lit.

Here, the case where a 4-bit grayscale is expressed has been described;however, the number of bits and the number of grayscales are not limitedthereto. Note that lighting is not needed to be performed in order ofTs1, Ts2, Ts3, and Ts4, and the order may be random or light emissionmay be performed in the period divided into a plurality of periods. Aratio of lighting times of Ts1, Ts2, Ts3, and Ts4 is not needed to bepower-of-two, and may be the same length or slightly different from apower-of-two.

Next, a driving method in which a signal writing period (address period)to a pixel and a light-emitting period (sustain period) are not dividedis described. A pixel in a row in which a writing operation of a videosignal is completed maintains the signal until another signal is writtento the pixel (or the signal is erased). Data holding time is a periodbetween the writing operation and the next writing operation of anothersignal to the pixel. In the data holding time, the pixel is lit or notlit in accordance with the video signal written to the pixel. The sameoperations are performed to the last row, and the address period ends.Then, an operation proceeds to a signal writing operation in a nextsubframe period sequentially from a row in which the data holding timeends.

As described above, in the case of a driving method in which a pixel islit or not lit in accordance with a video signal written to the pixelimmediately after the signal writing operation is completed and the dataholding time starts, signals cannot be input to two rows at the sametime, even if the data holding time is desired to be shorter than theaddress period. Accordingly, address periods need to be prevented fromoverlapping with each other. Therefore, the data holding time cannot bemade shorter than the address period. As a result, it becomes difficultto perform high-level grayscale display.

Thus, the data holding time is set to be shorter than the address periodby providing an erasing period. FIG. 74A illustrates a driving method inwhich the data holding time is set shorter than the address period byproviding an erasing period.

Here, the i-th pixel row is described with reference to FIG. 74B. In theaddress period Ta1, a pixel scan signal is input to a scan line in orderfrom a first row, and pixels are selected. Then, in the period Th1(i),while the pixels of the i-th row are selected, a video signal is inputto the pixels of the i-th row. Then, when the video signal is written tothe pixels of the i-th row, the pixels of the i-th row maintain thesignal until a signal is input again. Lighting and non-lighting of thepixels of the i-th row in the sustain period Ts1(i) are controlled bythe written video signal. That is, the pixels of the i-th row are lit ornot lit in accordance with the video signal written to the pixelsimmediately after the writing operation of the video signal to the i-throw is completed. Similarly, in the address periods Ta2, Ta3, and Ta4, avideo signal is input to the pixels of the i-th row, and lighting andnon-lighting of the pixels of the i-th row in the sustain periods Ts2,Ts3, and Ts4 are controlled by the video signal. Then, the end of asustain period Ts4(i) is set by the start of an erasing operation. Thisis because the pixels are forced to be not lit regardless of the videosignal written to the pixels of the i-th row in an erasing time Te(i).That is, the data holding time of the pixels of the i-th row ends whenthe erasing time Te(i) starts.

Thus, a display device with a high-level grayscale and a high duty ratio(ratio of a lighting period in one frame period), in which data holdingtime is shorter than an address period without separating the addressperiod and a sustain period, can be provided. Reliability of a displayelement can be improved since instantaneous luminance can be lowered.

Here, the case where a 4-bit grayscale is expressed has been described;however, the number of bits and the number of grayscales are not limitedthereto. Note that lighting is not needed to be performed in order ofTs1, Ts2, Ts3, and Ts4, and the order may be random or light emissionmay be performed in the period divided into a plurality of periods. Aratio of lighting time of Ts1, Ts2, Ts3, and Ts4 is not needed to bepower-of-two, and may be the same length or slightly different from apower of two.

A structure and an operation of a pixel to which digital time grayscaledriving can be applied are described.

FIG. 75 is a diagram showing an example of a pixel structure to whichdigital time grayscale driving can be applied.

A pixel 80300 includes a switching transistor 80301, a drivingtransistor 80302, a light-emitting element 80304, and a capacitor 80303.A gate of the switching transistor 80301 is connected to a scan line80306, a first electrode (one of a source electrode and a drainelectrode) of the switching transistor 80301 is connected to a signalline 80305, and a second electrode (the other of the source electrodeand the drain electrode) of the switching transistor 80301 is connectedto a gate of the driving transistor 80302. The gate of the drivingtransistor 80302 is connected to a power supply line 80307 through thecapacitor 80303, a first electrode of the driving transistor 80302 isconnected to the power supply line 80307, and a second electrode of thedriving transistor 80302 is connected to a first electrode (pixelelectrode) of the light-emitting element 80304. A second electrode ofthe light-emitting element 80304 corresponds to a common electrode80308.

The second electrode of the light-emitting element 80304 (the commonelectrode 80308) is set to a low power supply potential. The low powersupply potential is a potential satisfying (low power supplypotential)<(high power supply potential) with the high power supplypotential set to the power supply line 80307 as a reference. As the lowpower supply potential, GND, 0 V, or the like may be set, for example. Apotential difference between the high power supply potential and the lowpower supply potential is applied to the light-emitting element 80304,and a current is supplied to the light-emitting element 80304. Here, inorder to make the light-emitting element 80304 emit light, eachpotential is set so that the potential difference between the high powersupply potential and the low power supply potential is a forwardthreshold voltage or higher.

Gate capacitance of the driving transistor 80302 may be used as asubstitute for the capacitor 80303, so that the capacitor 80303 can beomitted. The gate capacitance of the driving transistor 80302 may beformed in a region where a source region, a drain region, an LDD region,or the like overlaps with the gate electrode. Alternatively, capacitancemay be formed between a channel region and the gate electrode.

In the case of voltage-input voltage driving method, a video signal isinput to the gate of the driving transistor 80302 so that the drivingtransistor 80302 is in either of two states of being sufficiently turnedon and turned off. That is, the driving transistor 80302 operates in alinear region.

The video signal such that the driving transistor 80302 operates in asaturation region is input, so that a current can be supplied to thelight-emitting element 80304. When the light-emitting element 80304 isan element luminance of which is determined in accordance with acurrent, luminance decay due to deterioration of the light-emittingelement 80304 can be suppressed. Further, when the video signal is ananalog signal, a current corresponding to the video signal can besupplied to the light-emitting element 80304. In this case, analoggrayscale driving can be performed.

A structure and an operation of a pixel called a threshold voltagecompensation pixel are described. A threshold voltage compensation pixelcan be applied to digital time grayscale driving and analog grayscaledriving.

FIG. 76 is a diagram showing an example of a structure of a pixel calleda threshold voltage compensation pixel.

The pixel in FIG. 76 includes a driving transistor 80600, a first switch80601, a second switch 80602, a third switch 80603, a first capacitor80604, a second capacitor 80605, and a light-emitting element 80620. Agate of the driving transistor 80600 is connected to a signal line 80611through the first capacitor 80604 and the first switch 80601 in thisorder. Further, the gate of the driving transistor 80600 is connected toa power supply line 80612 through the second capacitor 80605. A firstelectrode of the driving transistor 80600 is connected to the powersupply line 80612. A second electrode of the driving transistor 80600 isconnected to a first electrode of the light-emitting element 80606through the third switch 80603. Further, the second electrode of thedriving transistor 80600 is connected to the gate of the drivingtransistor 80600 through the second switch 80602. A second electrode ofthe light-emitting element 80606 corresponds to a common electrode80621. Note that whether the first switch 80601, the second switch80602, and the third switch 8003 are turned on or off are controlled bya signal input to a first scan line 80613, a signal input to a secondscan line 80615, and a signal input to a third scan line 80614,respectively.

A pixel structure shown in FIG. 76 is not limited thereto. For example,a switch, a resistor, a capacitor, a transistor, a logic circuit, or thelike may be added to the pixel in FIG. 76. For example, the secondswitch 80602 may include a p-channel transistor or an n-channeltransistor, the third switch 80603 may include a transistor havingpolarity opposite to that of the second switch 80602, and the secondswitch 80602 and the third switch 80603 may be controlled by the samescan line.

A structure and an operation of a pixel called a current input pixel aredescribed. A current input pixel can be applied to digital grayscaledriving and analog grayscale driving.

FIG. 77 illustrates an example of a structure of a pixel called acurrent input type.

The pixel in FIG. 77 includes a driving transistor 80700, a first switch80701, a second switch 80702, a third switch 80703, a capacitor 80704,and a light-emitting element 80730. A gate of the driving transistor80700 is connected to a signal line 80711 through the second switch80702 and the first switch 80701 in this order. Further, the gate of thedriving transistor 80700 is connected to a power supply line 80712through the capacitor 80704. A first electrode of the driving transistor80700 is connected to the power supply line 80712. A second electrode ofthe driving transistor 80700 is connected to the signal line 80711through the first switch 80701. Further, the second electrode of thedriving transistor 80700 is connected to a first electrode of thelight-emitting element 80730 through the third switch 80703. A secondelectrode of the light-emitting element 80730 corresponds to a commonelectrode 80731.

A pixel structure shown in FIG. 77 is not limited thereto. For example,a switch, a resistor, a capacitor, a transistor, a logic circuit, or thelike may be added to the pixel in FIG. 77. For example, the first switch80701 may include a p-channel transistor or an n-channel transistor, thesecond switch 80702 may include a transistor with the same polarity asthat of the first switch 80701, and the first switch 80701 and thesecond switch 80702 may be controlled by the same scan line. The secondswitch 80702 may be provided between the gate of the driving transistor80700 and the signal line 80711. Note that whether the first switch80701, the second switch 80702, and the third switch 80703 are turned onor off are controlled by a signal input to a first scan line 80713, asignal input to a second scan line 80714, and a signal input to a thirdscan line 80715, respectively.

Although this embodiment mode has been described with reference tovarious drawings, the contents (or part of the contents) described ineach drawing can be freely applied to, combined with, or replaced withthe contents (or part of the contents) described in another drawing.Further, much more drawings can be formed by combining each part in theabove-described drawings with another part.

Similarly, the contents (or part of the contents) described in eachdrawing in this embodiment mode can be freely applied to, combined with,or replaced with the contents (or part of the contents) described in adrawing in another embodiment mode. Further, much more drawings can beformed by combining each part in the drawings in this embodiment modewith part of another embodiment mode.

Note that this embodiment mode has described just examples of embodying,slightly transforming, modifying, improving, describing in detail, orapplying the contents (or part of the contents) described in otherembodiment modes, an example of related part thereof, or the like.Therefore, the contents described in other embodiment modes can befreely applied to, combined with, or replaced with the contentsdescribed in this embodiment mode.

Embodiment Mode 16

Embodiment Mode 16 will describe a pixel structure of a display device.In particular, a pixel structure of a display device using an organic ELelement is described.

FIG. 78A illustrates an example of a top plan view (layout diagram) of apixel including two transistors. FIG. 78B illustrates an example of across-sectional view along X-X′ in FIG. 78A.

FIGS. 78A and 78B show a first transistor 60105, a first wiring 60106, asecond wiring 60107, a second transistor 60108, a third wiring 60111, anopposite electrode 60112, a capacitor 60113, a pixel electrode 60115, apartition wall 60116, an organic conductive film 60117, an organic thinfilm 60118, and a substrate 60119. Note that it is preferable that thefirst transistor 60105 be used as a switching transistor, the secondtransistor 60108 as a driving transistor, the first wiring 60106 as agate signal line, the second wiring 60107 as a source signal line, andthe third wiring 60111 as a current supply line.

A gate electrode of the first transistor 60105 is electrically connectedto the first wiring 60106, one of a source electrode and a drainelectrode of the first transistor 60105 is electrically connected to thesecond wiring 60107, and the other of the source electrode or the drainelectrode of the first transistor 60105 is electrically connected to agate electrode of the second transistor 60108 and one electrode of thecapacitor 60113. Note that the gate electrode of the first transistor60105 includes a plurality of gate electrodes. Accordingly, a leakagecurrent in the off state of the first transistor 60105 can be reduced.

One of a source electrode and a drain electrode of the second transistor60108 is electrically connected to the third wiring 60111, and the otherof the source electrode and the drain electrode of the second transistor60108 is electrically connected to the pixel electrode 60115.Accordingly, a current flowing to the pixel electrode 60115 can becontrolled by the second transistor 60108.

The organic conductive film 60117 is provided over the pixel electrode60115, and the organic thin film 60118 (an organic compound layer) isfurther provided thereover. The opposite electrode 60112 is providedover the organic thin film 60118 (the organic compound layer). Note thatthe opposite electrode 60112 may be formed such that all pixels arecommonly connected, or may be patterned using a shadow mask or the like.

Light emitted from the organic thin film 60118 (the organic compoundlayer) is transmitted through either the pixel electrode 60115 or theopposite electrode 60112.

In FIG. 78B, a case where light is emitted to the pixel electrode side,that is, a side on which the transistors and the like are formed isreferred to as bottom emission; and a case where light is emitted to theopposite electrode side is referred to as top emission.

In the case of bottom emission, it is preferable that the pixelelectrode 60115 be formed of a transparent conductive film. In the caseof top emission, it is preferable that the opposite electrode 60112 beformed of a transparent conductive film.

In a light-emitting device for color display, EL elements havingrespective light emission colors of RGB may be separately formed, or anEL element with a single color may be formed over an entire surfaceuniformly and light emission of RGB can be obtained by using a colorfilter.

Note that the structure shown in FIGS. 78A and 78B is only an example,and various structures can be employed for a pixel layout, across-sectional structure, a stacking order of electrodes of an ELelement, and the like, as well as the structure shown in FIGS. 78A and78B. Further, as a light-emitting element, various elements such as acrystalline element such as an LED, and an element formed of aninorganic thin film can be used as well as the element formed of theorganic thin film shown in the drawing.

Although this embodiment mode has been described with reference tovarious drawings, the contents (or part of the contents) described ineach drawing can be freely applied to, combined with, or replaced withthe contents (or part of the contents) described in another drawing.Further, much more drawings can be formed by combining each part in theabove-described drawings with another part.

Similarly, the contents (or part of the contents) described in eachdrawing in this embodiment mode can be freely applied to, combined with,or replaced with the contents (or part of the contents) described in adrawing in another embodiment mode. Further, much more drawings can beformed by combining each part in the drawings in this embodiment modewith part of another embodiment mode.

Note that this embodiment mode has described just examples of embodying,slightly transforming, modifying, improving, describing in detail, orapplying the contents (or part of the contents) described in otherembodiment modes, an example of related part thereof, or the like.Therefore, the contents described in other embodiment modes can befreely applied to, combined with, or replaced with the contentsdescribed in this embodiment mode.

Embodiment Mode 17

This embodiment mode describes a structure of an EL element, particular,a structure of an organic EL element.

A structure of a mixed junction EL element is described. As an example,a structure is described, which includes a layer (a mixed layer) inwhich a plurality of materials among a hole injecting material, a holetransporting material, a light-emitting material, an electrontransporting material, an electron injecting material, and the like aremixed (hereinafter referred to as a mixed junction type EL element),which is different from a stacked-layer structure where a hole injectinglayer formed of a hole injecting material, a hole transporting layerformed of a hole transporting material, a light-emitting layer formed ofa light-emitting material, an electron transporting layer formed of anelectron transporting material, an electron injecting layer formed of anelectron injecting material, and the like are clearly distinguished.

FIGS. 79A to 79E are schematic views each showing a structure of a mixedjunction type EL element. Note that a layer interposed between an anode190101 and a cathode 190102 corresponds to an EL layer.

In the structure shown in FIG. 79A, the EL layer includes a holetransporting region 190103 formed of a hole transporting material and anelectron transporting region 190104 formed of an electron transportingmaterial. The hole transporting region 190103 is located closer to theanode than to the electron transporting region 190104. A mixed region190105 including both the hole transporting material and the electrontransporting material is provided between the hole transporting region190103 and the electron transporting region 190104.

Along a direction from the anode 190101 to the cathode 190102, theconcentration of the hole transporting material in the mixed region190105 is decreased and the concentration of the electron transportingmaterial in the mixed region 190105 is increased.

Note that a concentration gradient can be freely set. For example, aratio of concentrations of each functional material may be changed (aconcentration gradient may be formed) in the mixed region 190105including both the hole transporting material and the electrontransporting material, without including the hole transporting layer190103 formed of only the hole transporting material. Alternatively, aratio of concentrations of each functional material may be changed (aconcentration gradient may be formed) in the mixed region 190105including both the hole transporting material and the electrontransporting material, without including the hole transporting layer190103 formed of only the hole transporting material and the electrontransporting layer 190104 formed of only the electron transportingmaterial. Still alternatively, a ratio of concentrations may be changeddepending on a distance from the anode or the cathode. Note that theratio of concentrations may be changed continuously.

The mixed region 190105 includes a region 190106 to which alight-emitting material is added. A light emission color of the ELelement can be controlled by the light-emitting material. Further,carriers can be trapped by the light-emitting material. As thelight-emitting material, various fluorescent dyes as well as a metalcomplex having a quinoline skeleton, a benzoxazole skeleton, or abenzothiazole skeleton can be used. The light emission color of the ELelement can be controlled by adding the light-emitting material.

The anode 190101 is preferably formed using an electrode material havinga high work function in order to inject holes efficiently. For example,a light-transmitting electrode formed of indium tin oxide (ITO), indiumzinc oxide (IZO), ZnO, SnO₂, In₂O₃, or the like can be used. When alight-transmitting property is not needed, the anode 190101 may beformed of an opaque metal material.

As the hole transporting material, an aromatic amine compound or thelike can be used.

As the electron transporting material, a metal complex having aquinoline derivative, 8-quinolinol, or a derivative thereof as a ligand(especially tris(8-quinolinolato)aluminum (Alq₃)), or the like can beused.

The cathode 190102 is preferably formed using an electrode materialhaving a low work function in order to inject electrons efficiently. Forexample, a metal such as aluminum, indium, magnesium, silver, calcium,barium, or lithium can be used alone. Alternatively, an alloy of theaforementioned metal or an alloy of the aforementioned metal and anothermetal may be used.

FIG. 79B is the schematic view of the structure of the EL element, whichis different from that of FIG. 79A. Note that the same portions as thosein FIG. 79A are denoted by the same reference numerals, and thedescription is omitted.

In FIG. 79B, a region to which a light-emitting material is added is notprovided. However, light emission can be performed when a material(electron-transporting and light-emitting material) having both anelectron transporting property and a light-emitting property, forexample, tris(8-quinolinolato)aluminum (Alq₃) is used as a materialadded to the electron transporting region 190104.

Alternatively, as a material added to the hole transporting region190103, a material (a hole-transporting and light-emitting material)having both a hole transporting property and a light-emitting propertymay be used.

FIG. 79C is the schematic view of the structure of the EL element, whichis different from those of FIGS. 79A and 79B. Note that the sameportions as those in FIGS. 79A and 79B are denoted by the same referencenumerals, and the description is omitted.

In FIG. 79C, the mixed region 190105 includes a region 190107 to which ahole blocking material having a larger energy difference between thehighest occupied molecular orbital and the lowest unoccupied molecularorbital than the hole transporting material is added. When the region190107 to which the hole blocking material is added is located closer tothe cathode 190102 than to the region 190106 to which the light-emittingmaterial is added in the mixed region 190105, a recombination rate ofcarriers and light emission efficiency can be increased. Theaforementioned structure including the region 190107 to which the holeblocking material is added is especially effective in an EL elementwhich utilizes light emission (phosphorescence) by a triplet exciton.

FIG. 73D is the schematic view of the structure of the EL element, whichis different from those of FIGS. 73A to 73C. Note that the same portionsas those in FIGS. 79A to 79C are denoted by the same reference numerals,and the description is omitted.

In FIG. 79D, the mixed region 190105 includes a region 190108 to whichan electron blocking material having a larger energy difference betweenthe highest occupied molecular orbital and the lowest unoccupiedmolecular orbital than the electron transporting material is added. Whenthe region 190108 to which the electron blocking material is added islocated closer to the anode 190101 than to the region 190106 to whichthe light-emitting material is added in the mixed region 190105, arecombination rate of carriers and light emission efficiency can beincreased. The aforementioned structure including the region 190108 towhich the electron blocking material is added is especially effective inan EL element which utilizes light emission (phosphorescence) by atriplet exciton.

FIG. 79E is the schematic view of the structure of the mixed junctiontype EL element, which is different from those of FIGS. 79A to 79D. FIG.79E shows an example of a structure where the EL layer includes a region190109 to which a metal material is added in a portion in contact withan electrode of the EL element. In FIG. 79E, the same portions as thosein FIGS. 79A to 79D are denoted by the same reference numerals, and thedescription is omitted. In the structure shown in FIG. 79E, the cathode190102 may be formed using MgAg (an Mg—Ag alloy), and the electrontransporting region 190104 to which the electron transporting materialis added may include a region 190109 to which an aluminum (Al) alloy isadded in a region in contact with the cathode 190102. By employing theaforementioned structure, oxidation of the cathode can be prevented, andthe efficiency of electron injection from the cathode can be increased.Therefore, the lifetime of the mixed junction type EL element can beextended, and a driving voltage can be lowered.

As a method for forming the aforementioned mixed junction type ELelement, a co-evaporation method or the like can be used.

In the mixed junction type EL elements as shown in FIGS. 79A to 79E, adistinct interface between the layers does not exist, and chargeaccumulation can be reduced. Thus, the lifetime of the EL element can beextended, and a driving voltage can be lowered.

Note that the structures shown in FIGS. 79A to 79E can be implemented infree combination with each other.

The structure of the mixed junction type EL element is not limited tothose described above, and various structures can be freely used.

An organic material which is used to form an EL layer of an EL elementmay be a low molecular material, a high molecular material, or both ofthe materials. When a low molecular material is used as an organiccompound material, a film can be formed by an evaporation method. On theother hand, when a high molecular material is used for the EL layer, thehigh molecular material can be dissolved in a solvent and a film can beformed by a spin coating method or an ink-jet method.

The EL layer may be formed of an intermediate molecular material. Inthis specification, an intermediate molecule organic light-emittingmaterial refers to an organic light-emitting material without asublimation property and with a polymerization degree of approximately20 or less. When an intermediate molecular material is used for the ELlayer, a film can be formed by an ink-jet method or the like.

Note that a low molecular material, a high molecular material, and anintermediate molecular material may be used in combination.

An EL element may utilize either light emission (fluorescence) by asinglet exciton or light emission (phosphorescence) by a tripletexciton.

Although this embodiment mode has been described with reference tovarious drawings, the contents (or part of the contents) described ineach drawing can be freely applied to, combined with, or replaced withthe contents (or part of the contents) described in another drawing.Further, much more drawings can be formed by combining each part in theabove-described drawings with another part.

Similarly, the contents (or part of the contents) described in eachdrawing in this embodiment mode can be freely applied to, combined with,or replaced with the contents (or part of the contents) described in adrawing in another embodiment mode. Further, much more drawings can beformed by combining each part in the drawings in this embodiment modewith part of another embodiment mode.

Note that this embodiment mode has described just examples of embodying,slightly transforming, modifying, improving, describing in detail, orapplying the contents (or part of the contents) described in otherembodiment modes, an example of related part thereof, or the like.Therefore, the contents described in other embodiment modes can befreely applied to, combined with, or replaced with the contentsdescribed in this embodiment mode.

Embodiment Mode 18

This embodiment mode describes a structure of an EL element,particularly, a structure of an inorganic EL element.

As the base material used for a light-emitting material, a sulfide, anoxide, or a nitride can be used. As a sulfide, zinc sulfide (ZnS),cadmium sulfide (CdS), calcium sulfide (CaS), yttrium sulfide (Y₂S₃),gallium sulfide (Ga₂S₃), strontium sulfide (SrS), barium sulfide (BaS),or the like can be used, for example. As an oxide, zinc oxide (ZnO),yttrium oxide (Y₂O₃), or the like can be used, for example. As anitride, aluminum nitride (AlN), gallium nitride (GaN), indium nitride(InN), or the like can be used, for example. Further, zinc selenide(ZnSe), zinc telluride (ZnTe), or the like can also be used. A ternarymixed crystal such as calcium gallium sulfide (CaGa₂S₄), strontiumgallium sulfide (SrGa₂S₄), or barium gallium sulfide (BaGa₂S₄) may alsobe used.

As the light-emitting center of localized light emission, manganese(Mn), copper (Cu), samarium (Sm), terbium (Th), erbium (Er), thulium(Tm), europium (Eu), cerium (Ce), praseodymium (Pr), or the like can beused. Note that a halogen element such as fluorine (F) or chlorine (Cl)may be added as a charge compensation.

On the other hand, as the light-emitting center of donor-acceptorrecombination light emission, a light-emitting material which contains afirst impurity element forming a donor level and a second impurityelement forming an acceptor level can be used. As the first impurityelement, fluorine (F), chlorine (Cl), aluminum (Al), or the like can beused, for example. As the second impurity element, copper (Cu), silver(Ag), or the like can be used, for example.

FIGS. 80A to 80C each show an example of a thin-film type inorganic ELelement which can be used as a light-emitting element. In FIGS. 80A to80C, the light-emitting element includes a first electrode layer 120100,an electroluminescent layer 120102, and a second electrode layer 120103.

The light-emitting elements in FIGS. 80B and 80C each have a structurewhere an insulating film is provided between the electrode layer and theelectroluminescent layer in the light-emitting element in FIG. 80A. Thelight-emitting element in FIG. 80B includes an insulating film 120104between the first electrode layer 120100 and the electroluminescentlayer 120102. The light-emitting element in FIG. 80C includes aninsulating film 120105 between the first electrode layer 120100 and theelectroluminescent layer 120102, and an insulating film 120106 betweenthe second electrode layer 120103 and the electroluminescent layer120102. As described above, the insulating film may be provided betweenthe electroluminescent layer and one of the electrode layers sandwichingthe electroluminescent layer, or may be provided between theelectroluminescent layer and each of the electrode layers sandwichingthe electroluminescent layer. Further, the insulating film may be asingle layer or stacked layers including a plurality of layers.

FIGS. 81A to 81C each show an example of a dispersion type inorganic ELelement which can be used as a light-emitting element. Alight-emittingelement in FIG. 81A has a stacked-layer structure of a first electrodelayer 120200, an electroluminescent layer 120202, and a second electrodelayer 120203. The electroluminescent layer 120202 includes alight-emitting material 120201 held by a binder.

The light-emitting elements in FIGS. 81B and 81C each have a structurewhere an insulating film is provided between the electrode layer and theelectroluminescent layer in the light-emitting element in FIG. 81A. Thelight-emitting element in FIG. 81B includes an insulating film 120204between the first electrode layer 120200 and the electroluminescentlayer 120202. The light-emitting element in FIG. 81C includes aninsulating film 120205 between the first electrode layer 120200 and theelectroluminescent layer 120202, and an insulating film 120206 betweenthe second electrode layer 120203 and the electroluminescent layer120202. As described above, the insulating film may be provided betweenthe electroluminescent layer and one of the electrode layers sandwichingthe electroluminescent layer, or may be provided between theelectroluminescent layer and each of the electrode layers sandwichingthe electroluminescent layer. Further, the insulating film may be asingle layer or stacked layers including a plurality of layers.

The insulating film 120204 is provided in contact with the firstelectrode layer 120200 in FIG. 81B; however, the insulating film 120204may be provided in contact with the second electrode layer 120203 byreversing the positions of the insulating film and theelectroluminescent layer.

It is preferable that a material which can be used for the insulatingfilms such as the insulating film 120104 in FIG. 80B and the insulatingfilm 120204 in FIG. 81B has high withstand voltage and dense filmquality. Further, the material preferably has high dielectric constant.For example, silicon oxide (SiO₂), yttrium oxide (Y₂O₃), titanium oxide(TiO₂), aluminum oxide (Al₂O₃), hafnium oxide (HfO₂), tantalum oxide(Ta₂O₅), barium titanate (BaTiO₃), strontium titanate (SrTiO₃), leadtitanate (PbTiO₃), silicon nitride (Si₃N₄), or zirconium oxide (ZrO₂);or a mixed film of those materials or a stacked-layer film including twoor more of those materials can be used. The insulating film can beformed by sputtering, evaporation, CVD, or the like. Alternatively, theinsulating film may be formed by dispersing particles of theseinsulating materials in a binder. A binder material may be formed usinga material similar to that of a binder contained in theelectroluminescent layer, by using a method similar thereto. Thethickness of the insulating film is preferably, but not limited to, inthe range of 10 nm to 1000 nm.

Note that the light-emitting element can emit light when a voltage isapplied between the pair of electrode layers sandwiching theelectroluminescent layer. The light-emitting element can operate with DCdrive or AC drive.

Although this embodiment mode has been described with reference tovarious drawings, the contents (or part of the contents) described ineach drawing can be freely applied to, combined with, or replaced withthe contents (or part of the contents) described in another drawing.Further, much more drawings can be formed by combining each part in theabove-described drawings with another part.

Similarly, the contents (or part of the contents) described in eachdrawing in this embodiment mode can be freely applied to, combined with,or replaced with the contents (or part of the contents) described in adrawing in another embodiment mode. Further, much more drawings can beformed by combining each part in the drawings in this embodiment modewith part of another embodiment mode.

Note that this embodiment mode has described just examples of embodying,slightly transforming, modifying, improving, describing in detail, orapplying the contents (or part of the contents) described in otherembodiment modes, an example of related part thereof, or the like.Therefore, the contents described in other embodiment modes can befreely applied to, combined with, or replaced with the contentsdescribed in this embodiment mode.

Embodiment Mode 19

This embodiment mode describes an example of a display device,particularly, the case where a display device is optically treated.

A rear projection display device 130100 in FIGS. 82A and 82B is providedwith a projector unit 130111, a mirror 130112, and a screen panel130101. The rear projection display device 130100 may also be providedwith a speaker 130102 and operation switches 130104. The projector unit130111 is provided in a lower portion of a housing 130110 of the rearprojection display device 130100 and projects light for projecting animage based on a video signal to the mirror 130112. The rear projectiondisplay device 130100 displays an image projected from behind the screenpanel 130101.

FIG. 83 shows a front projection display device 130200. The frontprojection display device 130200 is provided with the projector unit130111 and a projection optical system 130201. The projection opticalsystem 130201 projects an image to a screen or the like provided at thefront.

Hereinafter, a structure of the projector unit 130111 which is appliedto the rear projection display device 130100 in FIGS. 83A and 83B andthe front projection display device 130200 in FIG. 84 is described.

FIG. 84 shows a structure example of the projector unit 130111. Theprojector unit 130111 is provided with a light source unit 130301 and amodulation unit 130304. The light source unit 130301 is provided with alight source optical system 130303 including lenses and a light sourcelamp 130302. The light source lamp 130302 is stored in a housing so thatstray light is not scattered. As the light source lamp 130302, ahigh-pressure mercury lamp or a xenon lamp, for example, which can emita large amount of light is used. The light source optical system 130303is provided with an optical lens, a film having a light-polarizingfunction, a film for adjusting phase difference, an IR film, or the likeas appropriate. The light source unit 130301 is provided so that emittedlight is incident on the modulation unit 130304. The modulation unit130304 is provided with a plurality of display panels 130308, a colorfilter, a dichroic mirror 130305, a total reflection mirror 130306, aretardation plate 130307, a prism 130309, and a projection opticalsystem 130310. Light emitted from the light source unit 130301 is splitinto a plurality of optical paths by the dichroic mirror 130305.

In each optical path, a color filter which transmits light with apredetermined wavelength or wavelength band and the display panel 130308are provided. The transmissive display panel 130308 modulatestransmission light based on a video signal. Light of each colortransmitted through the display panel 130308 is incident on the prism130309, and an image is displayed on the screen through the projectionoptical system 130310. Note that a Fresnel lens may be provided betweenthe mirror and the screen. Light which has been emitted from theprojector unit 130111 and reflected by the mirror is converted intocollimated light by the Fresnel lens and the collimated light isprojected on the screen. Displacement of the collimated light between achief ray and an optical axis is preferably ±10° or less, and morepreferably, ±5° or less.

The projector unit 130111 shown in FIG. 85 is provided with reflectivedisplay panels 130407, 130408, and 130409.

The projector unit 130111 in FIG. 85 is provided with the light sourceunit 130301 and a modulation unit 130400. The light source unit 130301may have a structure similar to FIG. 84. Light from the light sourceunit 130301 is split into a plurality of optical paths by dichroicmirrors 130401 and 130402 and a total reflection mirror 130403 to beincident on polarization beam splitters 130404, 130405, and 130406. Thepolarization beam splitters 130404, 130405, and 130406 are providedcorresponding to the reflective display panels 130407, 130408, and130409 which correspond to respective colors. The reflective displaypanels 130407, 130408, and 130409 modulate reflected light based on avideo signal. Light of each color, which is reflected by the reflectivedisplay panels 130407, 130408, and 130409, is incident on a prism 130410to be composed, and projected through a projection optical system130411.

Of light emitted from the light source unit 130301, the dichroic mirror130401 transmits only light in a wavelength range of red and reflectslight in wavelength ranges of green and blue. Further, the dichroicmirror 130402 reflects only the light in the wavelength range of green.The light in the wavelength range of red, which is transmitted throughthe dichroic mirror 130401, is reflected by the total reflection mirror130403 and incident on the polarization beam splitter 130404. The lightin the wavelength range of blue is incident on the polarization beamsplitter 130405. The light in the wavelength range of green is incidenton the polarization beam splitter 130406. The polarization beamsplitters 130404, 130405, and 130406 have a function to split incidentlight into P-polarized light and S-polarized light and a function totransmit only P-polarized light. The reflective display panels 130407,130408, and 130409 polarize incident light based on a video signal.

Only the S-polarized light corresponding to each color is incident onthe reflective display panels 130407, 130408, and 130409 correspondingto each color. Note that the reflective display panels 130407, 130408,and 130409 may be liquid crystal panels. In this case, the liquidcrystal panel operates in an electrically controlled birefringence (ECB)mode. Liquid crystal molecules are vertically aligned at an angle to asubstrate. Accordingly, in the reflective display panels 130407, 130408,and 130409, when a pixel is turned off, display molecules are alignednot to change a polarization state of incident light so as to reflectthe incident light. When the pixel is turned on, alignment of thedisplay molecules is changed, and the polarization state of the incidentlight is changed.

The projector unit 130111 in FIG. 85 can be applied to the rearprojection display device 130100 in FIGS. 82A and 82B and the frontprojection display device 130200 in FIG. 83.

FIGS. 86A to 86C each show a single-panel type projector unit. Theprojector unit 130111 shown in FIG. 86A is provided with the lightsource unit 130301, a display panel 130507, a projection optical system130511, and a retardation plate 130504. The projection optical system130511 includes one or a plurality of lenses. The display panel 130507may be provided with a color filter.

FIG. 86B shows a structure of the projector unit 130111 operating in afield sequential mode. The field sequential mode corresponds to a modein which color display is performed by light of respective colors suchas red, green, and blue sequentially incident on a display panel with atime lag, without a color filter. A high-definition image can bedisplayed particularly by combination with a display panel withhigh-speed response to a change in input signal. The projector unit130111 in FIG. 80B is provided with a rotating color filter plate 130505including a plurality of color filters with red, green, blue, or thelike between the light source unit 130301 and a display panel 130508.

FIG. 86C shows a structure of the projector unit 130111 with a colorseparation system using a micro lens, as a color display method. Thecolor separation system corresponds to a system in which color displayis realized by providing a micro lens array 130506 on the side of adisplay panel 130509, on which light is incident, and light of eachcolor is emitted from each direction. The projector unit 130111employing this system has little loss of light due to a color filter, sothat light from the light source unit 130301 can be efficientlyutilized. The projector unit 130111 in FIG. 86C is provided withdichroic mirrors 130501, 130502, and 130503 so that light of each coloris emitted to the display panel 130509 from each direction.

Although this embodiment mode has been described with reference tovarious drawings, the contents (or part of the contents) described ineach drawing can be freely applied to, combined with, or replaced withthe contents (or part of the contents) described in another drawing.Further, much more drawings can be formed by combining each part in theabove-described drawings with another part.

Similarly, the contents (or part of the contents) described in eachdrawing in this embodiment mode can be freely applied to, combined with,or replaced with the contents (or part of the contents) described in adrawing in another embodiment mode. Further, much more drawings can beformed by combining each part in the drawings in this embodiment modewith part of another embodiment mode.

Note that this embodiment mode has described just examples of embodying,slightly transforming, modifying, improving, describing in detail, orapplying the contents (or part of the contents) described in otherembodiment modes, an example of related part thereof, or the like.Therefore, the contents described in other embodiment modes can befreely applied to, combined with, or replaced with the contentsdescribed in this embodiment mode.

Embodiment Mode 20

Embodiment Mode 20 will describe examples of electronic devices.

FIG. 87 illustrates a display panel module combining a display panel900101 and a circuit board 900111. The display panel 900101 includes apixel portion 900102, a scan line driver circuit 900103, and a signalline driver circuit 900104. The circuit board 900111 is provided with acontrol circuit 900112, a signal dividing circuit 900113, and the like,for example. The display panel 900101 and the circuit board 900111 areconnected to each other by a connection wiring 900114. An FPC or thelike can be used as the connection wiring.

FIG. 92 is a block diagram of a main structure of a television receiver.A tuner 900201 receives a video signal and an audio signal. The videosignals are processed by a video signal amplifier circuit 900202; avideo signal processing circuit 900203 and a control circuit 900212. Thevideo signal processing circuit 900203 converts a signal output from thevideo signal amplifier circuit 900202 into a color signal correspondingto each color of red, green, and blue. The control circuit 900212converts the video signal into the input specification of a drivercircuit. The control circuit 900212 outputs a signal to each of a scanline driver circuit 900214 and a signal line driver circuit 900204. Thescan line driver circuit 900214 and the signal line driver circuit900204 drive a display panel 900211. When performing digital driving, astructure may be employed in which a signal dividing circuit 900213 isprovided on the signal line side so that an input digital signal isdivided into m signals (m is a positive integer) to be supplied.

Among the signals received by the tuner 900201, an audio signal istransmitted to an audio signal amplifier circuit 900205, and an outputthereof is supplied to a speaker 900207 through an audio signalprocessing circuit 900206. A control circuit 900208 receives controlinformation on receiving station (receiving frequency) and volume froman input portion 900209 and transmits signals to the tuner 900201 or theaudio signal processing circuit 900206.

FIG. 93A illustrates a television receiver incorporated with a displaypanel module, which is different from FIG. 92. In FIG. 93A, a displayscreen 900302 incorporated in a housing 900301 is formed using thedisplay panel module. Note that speakers 900303, input means (anoperation key 900304, a connection terminal 900305, a sensor 900306(having a function to measure power, displacement, position, speed,acceleration, angular velocity, the number of rotations, distance,light, liquid, magnetism, temperature, a chemical substance, sound,time, hardness, an electric field, current, voltage, electric power,radiation, a flow rate, humidity, gradient, oscillation, smell, orinfrared ray), and a microphone 900307), and the like may be provided asappropriate.

FIG. 93B illustrates a television receiver in which a display can becarried wirelessly. The television receiver is provided with a displayportion 900313, a speaker portion 900317, input means (an operation key900316, a connection terminal 900318, a sensor 900319 (having a functionto measure power, displacement, position, speed, acceleration, angularvelocity, the number of rotations, distance, light, liquid, magnetism,temperature, a chemical substance, sound, time, hardness, an electricfield, current, voltage, electric power, radiation, a flow rate,humidity, gradient, oscillation, smell, or infrared ray), and amicrophone 900320), and the like as appropriate. A battery and a signalreceiver are incorporated in a housing 900312. The battery drives thedisplay portion 900313, the speaker portion 900317, the sensor 900319,and the microphone 900320. The battery can be repeatedly charged by acharger 900310. The charger 900310 can transmit and receive a videosignal and transmit the video signal to the signal receiver of thedisplay. The device in FIG. 93B is controlled by the operation key900316. Alternatively, the device in FIG. 93B can transmit a signal tothe charger 900310 by operating the operation key 900316. That is, thedevice may be a video-audio two-way communication device. Furtheralternatively, by operating the operation key 900316, the device in FIG.93B may transmit a signal to the charger 900310 and make anotherelectronic device receive a signal which can be transmitted from thecharger 900310; thus, the device in FIG. 93B can control communicationof another electronic device. That is, the device may be ageneral-purpose remote control device. Note that the contents (or partthereof) described in each drawing of this embodiment mode can beapplied to the display portion 900313.

Next, a structure example of a mobile phone is described with referenceto FIG. 94.

A display panel 900501 is detachably incorporated in a housing 900530.The shape and size of the housing 900530 can be changed as appropriatein accordance with the size of the display panel 900501. The housing900530 which fixes the display panel 900501 is fitted in a printedwiring board 900531 to be assembled as a module.

The display panel 900501 is connected to the printed wiring board 900531through an FPC 900513. The printed wiring board 900531 is provided witha speaker 900532, a microphone 900533, a transmitting/receiving circuit900534, a signal processing circuit 900535 including a CPU, acontroller, and the like, and a sensor 900541 (having a function tomeasure power, displacement, position, speed, acceleration, angularvelocity, the number of rotations, distance, light, liquid, magnetism,temperature, a chemical substance, sound, time, hardness, an electricfield, current, voltage, electric power, radiation, a flow rate,humidity, gradient, oscillation, smell, or infrared ray). Such a module,an operation key 900536, a battery 900537, and an antenna 900540 arecombined and stored in a housing 900539. A pixel portion of the displaypanel 900501 is provided to be viewed from an opening window formed inthe housing 900539.

In the display panel 900501, the pixel portion and part of peripheraldriver circuits (a driver circuit having a low operation frequency amonga plurality of driver circuits) may be formed over the same substrate byusing transistors, and another part of the peripheral driver circuits (adriver circuit having a high operation frequency among the plurality ofdriver circuits) may be formed over an IC chip. Then, the IC chip may bemounted on the display panel 900501 by COG (Chip On Glass).Alternatively, the IC chip may be connected to a glass substrate byusing TAB (Tape Automated Bonding) or a printed wiring board. With sucha structure, power consumption of a display device can be reduced andoperation time of the mobile phone per charge can be extended. Further,reduction in cost of the mobile phone can be realized.

The mobile phone in FIG. 94 has various functions such as, but notlimited to, a function to display various kinds of information (e.g., astill image, a moving image, and a text image); a function to display acalendar, a date, the time, and the like on a display portion; afunction to operate or edit the information displayed on the displayportion; a function to control processing by various kinds of software(programs); a function of wireless communication; a function tocommunicate with another mobile phone, a fixed phone, or an audiocommunication device by using the wireless communication function; afunction to connect with various computer networks by using the wirelesscommunication function; a function to transmit or receive various kindsof data by using the wireless communication function; a function tooperate a vibrator in accordance with incoming call, reception of data,or an alarm; and a function to produce a sound in accordance withincoming call, reception of data, or an alarm.

FIG. 95A illustrates a display, which includes a housing 900711, asupport base 900712, a display portion 900713, a speaker 900717, an LEDlamp 900719, input means (a connection terminal 900714, a sensor 900715(having a function to measure power, displacement, position, speed,acceleration, angular velocity, the number of rotations, distance,light, liquid, magnetism, temperature, a chemical substance, sound,time, hardness, an electric field, current, voltage, electric power,radiation, a flow rate, humidity, gradient, oscillation, smell, orinfrared ray), a microphone 900716, and an operation key 900718), andthe like. The display in FIG. 95A can have various functions such as,but not limited to, a function to display various kinds of information(e.g., a still image, a moving image, and a text image) on the displayportion.

FIG. 95B illustrates a camera, which includes a main body 900731, adisplay portion 900732, a shutter button 900736, a speaker 900740, anLED lamp 900741, input means (an image receiving portion 900733,operation keys 900734, an external connection port 900735, a connectionterminal 900737, a sensor 900738 (having a function to measure power,displacement, position, speed, acceleration, angular velocity, thenumber of rotations, distance, light, liquid, magnetism, temperature, achemical substance, sound, time, hardness, an electric field, current,voltage, electric power, radiation, a flow rate, humidity, gradient,oscillation, smell, or infrared ray), and a microphone 900739), and thelike. The camera in FIG. 95B can have various functions such as, but notlimited to, a function to photograph a still image or a moving image; afunction to automatically adjust the photographed image (still image ormoving image); a function to store the photographed image in a recordingmedium (provided externally or incorporated in the camera); and afunction to display the photographed image on the display portion.

FIG. 95C illustrates a computer, which includes a main body 900751, ahousing 900752, a display portion 900753, a speaker 900760, an LED lamp900761, a reader/writer 900762, input means (a keyboard 900754, anexternal connection port 900755, a pointing device 900756, a connectionterminal 900757, a sensor 900758 (having a function to measure power,displacement, position, speed, acceleration, angular velocity, thenumber of rotations, distance, light, liquid, magnetism, temperature, achemical substance, sound, time, hardness, an electric field, current,voltage, electric power, radiation, a flow rate, humidity, gradient,oscillation, smell, or infrared ray), and a microphone 900759), and thelike. The computer in FIG. 95C can have various functions such as, butnot limited to, a function to display various kinds of information(e.g., a still image, a moving image, and a text image) on the displayportion; a function to control processing by various kinds of software(programs); a communication function such as wireless communication orwire communication; a function to connect with various computer networksby using the communication function; and a function to transmit orreceive various kinds of data by using the communication function.

FIG. 102A illustrates a mobile computer, which includes a main body901411, a display portion 901412, a switch 901413, a speaker 901419, anLED lamp 901420, input means (operation keys 901414, an infrared port901415, a connection terminal 901416, a sensor 901417 (having a functionto measure power, displacement, position, speed, acceleration, angularvelocity, the number of rotations, distance, light, liquid, magnetism,temperature, a chemical substance, sound, time, hardness, an electricfield, current, voltage, electric power, radiation, a flow rate,humidity, gradient, oscillation, smell, or infrared ray), and amicrophone 901418), and the like. The mobile computer in FIG. 102A canhave various functions such as, but not limited to, a function todisplay various kinds of information (e.g., a still image, a movingimage, and a text image) on the display portion; a touch panel functionprovided on the display portion; a function to display a calendar, adate, the time, and the like on the display portion; a function tocontrol processing by various kinds of software (programs); a functionof wireless communication; a function to connect with various computernetworks by using the wireless communication function; and a function totransmit or receive various kinds of data by using the wirelesscommunication function.

FIG. 102B illustrates a portable image reproducing device having arecording medium (e.g., a DVD player), which includes a main body901431, a housing 901432, a display portion A 901433, a display portionB 901434, a speaker portion 901437, an LED lamp 901441, input means (arecording medium (e.g., DVD) reading portion 901435, operation keys901436, a connection terminal 901438, a sensor 901439 (having a functionto measure power, displacement, position, speed, acceleration, angularvelocity, the number of rotations, distance, light, liquid, magnetism,temperature, a chemical substance, sound, time, hardness, an electricfield, current, voltage, electric power, radiation, a flow rate,humidity, gradient, oscillation, smell, or infrared ray), and amicrophone 901440), and the like. The display portion A 901433 mainlydisplays image information and the display portion B 901434 mainlydisplays text information.

FIG. 102C illustrates a goggle-type display, which includes a main body901451, a display portion 901452, an earphone 901453, a support portion901454, an LED lamp 901459, a speaker 901458, input means (a connectionterminal 901455, a sensor 901456 (having a function to measure power,displacement, position, speed, acceleration, angular velocity, thenumber of rotations, distance, light, liquid, magnetism, temperature, achemical substance, sound, time, hardness, an electric field, current,voltage, electric power, radiation, a flow rate, humidity, gradient,oscillation, smell, or infrared ray), and a microphone 901457), and thelike. The goggle-type display in FIG. 102C can have various functionssuch as, but not limited to, a function to display an externallyobtained image (e.g., a still image, a moving image, and a text image)on the display portion.

FIG. 103A illustrates a portable game machine, which includes a housing901511, a display portion 901512, a speaker portion 901513, a recordingmedium insert portion 901515, an LED lamp 901519, input means (anoperation key 901514, a connection terminal 901516, a sensor 901517(having a function to measure power, displacement, position, speed,acceleration, angular velocity, the number of rotations, distance,light, liquid, magnetism, temperature, a chemical substance, sound,time, hardness, an electric field, current, voltage, electric power,radiation, a flow rate, humidity, gradient, oscillation, smell, orinfrared ray), and a microphone 901518), and the like. The portable gamemachine in FIG. 103A can have various functions such as, but not limitedto, a function to read a program or data stored in the recording mediumto display on the display portion; and a function to share informationwith another portable game machine by wireless communication.

FIG. 103B illustrates a digital camera having a television receptionfunction, which includes a main body 901531, a display portion 901532, aspeaker 901534, a shutter button 901535, an LED lamp 901541, input means(an operation key 901533, an image receiving portion 901536, an antenna901537, a connection terminal 901538, a sensor 901539 (having a functionto measure power, displacement, position, speed, acceleration, angularvelocity, the number of rotations, distance, light, liquid, magnetism,temperature, a chemical substance, sound, time, hardness, an electricfield, current, voltage, electric power, radiation, a flow rate,humidity, gradient, oscillation, smell, or infrared ray), and amicrophone 901540), and the like. The digital camera having a televisionreception function in FIG. 103B can have various functions such as, butnot limited to, a function to photograph a still image or a movingimage; a function to automatically adjust the photographed image; afunction to obtain various kinds of information from the antenna; afunction to store the photographed image or the information obtainedfrom the antenna; and a function to display the photographed image orthe information obtained from the antenna on the display portion.

FIG. 104 illustrates a portable game machine, which includes a housing901611, a first display portion 901612, a second display portion 901613,a speaker portion 901614, a recording medium insert portion 901616, anLED lamp 901620, input means (an operation key 901615, a connectionterminal 901617, a sensor 901618 (having a function to measure power,displacement, position, speed, acceleration, angular velocity, thenumber of rotations, distance, light, liquid, magnetism, temperature, achemical substance, sound, time, hardness, an electric field, current,voltage, electric power, radiation, a flow rate, humidity, gradient,oscillation, smell, or infrared ray), and a microphone 901619), and thelike. The portable game machine in FIG. 104 can have various functionssuch as, but not limited to, a function to read a program or data storedin the recording medium to display on the display portion; and afunction to share information with another portable game machine bywireless communication.

As shown in FIGS. 95A to 95C, 102A to 102C, 103A, 103B, and 104, theelectronic devices include a display portion for displaying some kind ofinformation.

Next, application examples of a semiconductor device are described.

FIG. 96 illustrates an example where a semiconductor device isincorporated in a constructed object. FIG. 96 illustrates a housing900810, a display portion 900811, a remote control device 900812 whichis an operation portion, a speaker portion 900813, and the like. Thesemiconductor device is attached to or incorporated in the constructedobject as a wall-hanging type and can be provided without requiring alarge space.

FIG. 97 illustrates another example where a semiconductor device isincorporated in a constructed object. A display panel 900901 isincorporated with a prefabricated bath 900902, and a person who takes abath can view the display panel 900901. The display panel 900901 has afunction to display information by an operation by a person who takes abath; and a function to be used as an advertisement or an entertainmentmeans.

The semiconductor device can be provided not only to a side wall of theprefabricated bath 900902 as shown in FIG. 97, but also to variousplaces. For example, the semiconductor device can be attached to orunified with part of a mirror, a bathtub itself, or the like. At thistime, the shape of the display panel 900901 may be changed in accordancewith the shape of the mirror or the bathtub.

FIG. 98 illustrates another example where a semiconductor device isunified with a constructed object. A display panel 901002 is bent andattached to a curved surface of a column-shaped object 901001. Here, autility pole is described as the column-shaped object 901001.

The display panel 901002 in FIG. 98 is provided at a position higherthan a human viewpoint. When the display panels 901002 are provided inconstructed objects which stand together in large numbers outdoors, suchas utility poles, advertisement to unspecified number of viewers can beperformed. Since it is easy for the display panel 901002 to display thesame images and instantly switch images by external control, highlyeffective information display and advertisement effect can be expected.When provided with self-luminous display elements, the display panel901002 can be effectively used as a highly visible display medium evenat night. When the display panel 901002 is provided in the utility pole,a power supply means for the display panel 901002 can be easilyobtained. In an emergency such as disaster, the display panel 901002 canalso rapidly transmit correct information to victims.

An example of the display panel 901002 is a display panel which displaysan image by driving a display element with a switching element such asan organic transistor provided over a film-like substrate.

In this embodiment mode, a wall, a column-shaped object, and aprefabricated bath are shown as examples of a constructed object;however, this embodiment mode is not limited thereto, and variousconstructed objects can be provided with a semiconductor device.

Next, examples where a semiconductor device is incorporated with amoving object are described.

FIG. 99 illustrates an example where a semiconductor device isincorporated with a car. A display panel 901102 is incorporated with acar body 901101, and can display an operation of the car body orinformation input from inside or outside the car body on demand. Notethat a navigation function may be provided.

The semiconductor device can be provided not only to the car body 901101as shown in FIG. 99, but also to various places. For example, thesemiconductor device can be incorporated with a glass window, a door, asteering wheel, a gear shift, a seat, a rear-view mirror, and the like.At this time, the shape of the display panel 901102 may be changed inaccordance with the shape of an object provided with the semiconductordevice.

FIGS. 100A and 100B show examples where a semiconductor device isincorporated with a train car are described.

FIG. 100A illustrates an example where a display panel 901202 isprovided in glass of a door 901201 in a train car, which has anadvantage compared with a conventional advertisement using paper in thatlabor cost for changing an advertisement is not necessary. Since thedisplay panel 901202 can instantly switch images displayed on a displayportion by an external signal, images on the display panel can beswitched every time period when types of passengers on the train arechanged, for example; thus, more effective advertisement effect can beexpected.

FIG. 100B illustrates an example where the display panels 901202 areprovided to a glass window 901203 and a ceiling 901204 as well as theglass of the door 901201 in the train car. In this manner, thesemiconductor device can be easily provided to a place where thesemiconductor device has been difficult to be provided conventionally;thus, effective advertisement effect can be obtained. Further, thesemiconductor device can instantly switch images displayed on a displayportion by an external signal; thus, cost and time for changing anadvertisement can be reduced, and more flexible advertisement managementand information transmission can be realized.

The semiconductor device can be provided not only to the door 901201,the glass window 901203, and the ceiling 901204 as shown in FIG. 100,but also to various places. For example, the semiconductor device can beincorporated with a strap, a seat, a handrail, a floor, and the like. Atthis time, the shape of the display panel 901202 may be changed inaccordance with the shape of an object provided with the semiconductordevice.

FIGS. 101A and 101B show an example where a semiconductor device isincorporated with a passenger airplane.

FIG. 101A illustrates the shape of a display panel 901302 attached to aceiling 901301 above a seat of the passenger airplane when the displaypanel 901302 is used. The display panel 901302 is incorporated with theceiling 901301 using a hinge portion 901303, and the passenger can viewthe display panel 901302 by stretching of the hinge portion 901303. Thedisplay panel 901302 has a function to display information by anoperation by the passenger and a function to be used as an advertisementor an entertainment means. When the hinge portion is bent and put in theceiling 901301 of the airplane as shown in FIG. 101B, safety intaking-off and landing can be assured. Note that when a display elementin the display panel is lit in an emergency, the display panel can alsobe used as an information transmission means and an evacuation light.

The semiconductor device can be provided not only to the ceiling 901301as shown in FIGS. 101A and 101B, but also to various places. Forexample, the semiconductor device can be incorporated with a seat, atable attached to a seat, an armrest, a window, and the like. Alarge-scale display panel which a large number of people can view may beprovided at a wall of an airframe. At this time, the shape of thedisplay panel 901302 may be changed in accordance with the shape of anobject provided with the semiconductor device.

Note that in this embodiment mode, bodies of a train car, a car, and anairplane are shown as a moving object; however, the present invention isnot limited thereto, and a semiconductor device can be provided tovarious objects such as a motorcycle, an four-wheel drive car (includinga car, a bus, and the like), a train (including a monorail, a railroadcar, and the like), and a vessel. Since a semiconductor device caninstantly switch images displayed on a display panel in a moving objectby an external signal, a moving object is provided with thesemiconductor device, so that the moving object can be used as anadvertisement display board for an unspecified number of customers, aninformation display board in disaster, and the like.

Although this embodiment mode has been described with reference tovarious drawings, the contents (or part of the contents) described ineach drawing can be freely applied to, combined with, or replaced withthe contents (or part of the contents) described in another drawing.Further, much more drawings can be formed by combining each part in theabove-described drawings with another part.

Similarly, the contents (or part of the contents) described in eachdrawing in this embodiment mode can be freely applied to, combined with,or replaced with the contents (or part of the contents) described in adrawing in another embodiment mode. Further, much more drawings can beformed by combining each part in the drawings in this embodiment modewith part of another embodiment mode.

Note that this embodiment mode has described just examples of embodying,slightly transforming, modifying, improving, describing in detail, orapplying the contents (or part of the contents) described in otherembodiment modes, an example of related part thereof, or the like.Therefore, the contents described in other embodiment modes can befreely applied to, combined with, or replaced with the contentsdescribed in this embodiment mode.

Embodiment Mode 21

As described above, the present invention includes at least thefollowing aspects.

One aspect is a display device including a pixel portion having aplurality of pixels and a driver circuit electrically connected to thepixel portion. The driver circuit includes a first transistor, a secondtransistor, a third transistor, a fourth transistor, a fifth transistor,a sixth transistor, and a seventh transistor. This driver circuit atleast partly has the following connection relationship. A firstelectrode of the first transistor is electrically connected to a fourthwiring, and a second electrode of the first transistor is electricallyconnected to a third wiring. A first electrode of the second transistoris electrically connected to a sixth wiring, and a second electrode ofthe second transistor is electrically connected to the third wiring. Afirst electrode of the third transistor is electrically connected to afifth wiring, a second electrode of the third transistor is electricallyconnected to a gate electrode of the second transistor, and a gateelectrode of the third transistor is electrically connected to a seventhwiring. A first electrode of the fourth transistor is electricallyconnected to the sixth wiring, a second electrode of the fourthtransistor is electrically connected to the gate electrode of the secondtransistor, and a gate electrode of the fourth transistor iselectrically connected to a gate electrode of the first transistor. Afirst electrode of the fifth transistor is electrically connected to theseventh wiring, a second electrode of the fifth transistor iselectrically connected to the gate electrode of the first transistor,and a gate electrode of the fifth transistor is electrically connectedto a first wiring. A first electrode of the sixth transistor iselectrically connected to the sixth wiring, a second electrode of thesixth transistor is electrically connected to the gate electrode of thefirst transistor, and a gate electrode of the sixth transistor iselectrically connected to the gate electrode of the second transistor. Afirst electrode of the seventh transistor is electrically connected tothe sixth wiring, a second electrode of the seventh transistor iselectrically connected to the gate electrode of the first transistor,and a gate electrode of the seventh transistor is electrically connectedto a second wiring.

The display device including the pixel portion having a plurality ofpixels and the driver circuit electrically connected to the pixelportion may have the following feature. A feature is that a value ofratio W/L of a channel length Land a channel width W of the firsttransistor is the greatest among values of ratios W/L of the first toseventh transistors. Another feature is that a value of ratio W/L of achannel length L and a channel width W of the first transistor is two tofive times greater than a value of ratio W/L of the fifth transistor.Another feature is that a channel length L of the third transistor isgreater than a channel length of the fourth transistor. Another featureis that a capacitor is provided between the second electrode of thefirst transistor and the gate electrode of the first transistor. Anotherfeature is that each of the first to seventh transistors is an n-channeltransistor. Another feature is that each of the first to seventhtransistors uses amorphous silicon as a semiconductor layer.

Another aspect is a display device including a pixel portion having aplurality of pixels, and a first driver circuit and a second drivercircuit electrically connected to the pixel portion. The first drivercircuit and the second driver circuit at least partly has the followingconnection relationship. The first driver circuit includes a firsttransistor, a second transistor, a third transistor, a fourthtransistor, a fifth transistor, a sixth transistor, and a seventhtransistor. A first electrode of the first transistor is electricallyconnected to a fourth wiring, and a second electrode of the firsttransistor is electrically connected to a third wiring. A firstelectrode of the second transistor is electrically connected to a sixthwiring, and a second electrode of the second transistor is electricallyconnected to the third wiring. A first electrode of the third transistoris electrically connected to a fifth wiring, a second electrode of thethird transistor is electrically connected to a gate electrode of thesecond transistor, and a gate electrode of the third transistor iselectrically connected to a seventh wiring. A first electrode of thefourth transistor is electrically connected to the sixth wiring, asecond electrode of the fourth transistor is electrically connected tothe gate electrode of the second transistor, and a gate electrode of thefourth transistor is electrically connected to a gate electrode of thefirst transistor. A first electrode of the fifth transistor iselectrically connected to the seventh wiring, a second electrode of thefifth transistor is electrically connected to the gate electrode of thefirst transistor, and a gate electrode of the fifth transistor iselectrically connected to a first wiring. A first electrode of the sixthtransistor is electrically connected to the sixth wiring, a secondelectrode of the sixth transistor is electrically connected to the gateelectrode of the first transistor, and a gate electrode of the sixthtransistor is electrically connected to the gate electrode of the secondtransistor. A first electrode of the seventh transistor is electricallyconnected to the sixth wiring, a second electrode of the seventhtransistor is electrically connected to the gate electrode of the firsttransistor, and a gate electrode of the seventh transistor iselectrically connected to a second wiring. The second driver circuitincludes an eighth transistor, a ninth transistor, a tenth transistor,an eleventh transistor, a twelfth transistor, a thirteenth transistor,and a fourteenth transistor. A first electrode of the eighth transistoris electrically connected to a eleventh wiring, and a second electrodeof the eighth transistor is electrically connected to a tenth wiring. Afirst electrode of the ninth transistor is electrically connected to athirteenth wiring, and a second electrode of the ninth transistor iselectrically connected to the tenth wiring. A first electrode of thetenth transistor is electrically connected to a twelfth wiring, a secondelectrode of the tenth transistor is electrically connected to a gateelectrode of the ninth transistor, and a gate electrode of the tenthtransistor is electrically connected to a fourteenth wiring. A firstelectrode of the eleventh transistor is electrically connected to thethirteenth wiring, a second electrode of the eleventh transistor iselectrically connected to the gate electrode of the ninth transistor,and a gate electrode of the eleventh transistor is electricallyconnected to a gate electrode of the eighth transistor. A firstelectrode of the twelfth transistor is electrically connected to the14th wiring, wherein a second electrode of the twelfth transistor iselectrically connected to the gate electrode of the eighth transistor,wherein a gate electrode of the twelfth transistor is electricallyconnected to an eighth wiring. A first electrode of the thirteenthtransistor is electrically connected to the thirteenth wiring, a secondelectrode of the thirteenth transistor is electrically connected to thegate electrode of the eighth transistor, and a gate electrode of thethirteenth transistor is electrically connected to the gate electrode ofthe ninth transistor. A first electrode of the fourteenth transistor iselectrically connected to the thirteenth wiring, a second electrode ofthe fourteenth transistor is electrically connected to the gateelectrode of the eighth transistor, and a gate electrode of thefourteenth transistor is electrically connected to a ninth wiring.

The display device including the pixel portion having a plurality ofpixels, and the first driver circuit and the second driver circuitelectrically connected to the pixel portion may have the followingfeature. One feature is that the fourth wiring and the eleventh wiringare electrically connected to each other, the fifth wiring and thetwelfth wiring are electrically connected to each other, the sixthwiring and the thirteenth wiring are electrically connected to eachother, and the seventh wiring and the fourteenth wiring are electricallyconnected to each other. Another feature is that the fourth wiring andthe eleventh wiring are the same wiring, the fifth wiring and thetwelfth wiring are the same wiring, the sixth wiring and the thirteenthwiring are the same wiring, and the seventh wiring and the fourteenthwiring are the same wiring. Another feature is that the third wiring andthe tenth wiring are electrically connected to each other. Anotherfeature is that the third wiring and the tenth wiring are the samewiring. Another feature is that a value of ratio W/L of a channel lengthL and a channel width W of the first transistor is the greatest amongvalues of ratios W/L of the first to seventh transistors, and a value ofratio W/L of a channel length Land a channel width W of the eighthtransistor is the greatest among values of ratios W/L of the eighth tofourteenth transistors. Another feature is that the value of ratio W/Lof a channel length L and a channel width W of the first transistor istwo to five times greater than a value of W/L of the fifth transistor,and the value of ratio W/L of a channel length L and a channel width Wof the eighth transistor is two to five times greater than a value ofW/L of the twelfth transistor. Another feature is that a channel lengthL of the third transistor is larger than a channel length L of thefourth transistor, and a channel length L of the tenth transistor islarger than a channel length of the eleventh transistor. Another featureis that a capacitor is provided between the second electrode and thegate electrode of the first transistor, and a capacitor is providedbetween the second electrode and the gate electrode of the eighthtransistor. Another feature is that each of the first driver circuit andthe second driver circuit is a flip-flop circuit. Another feature isthat each of the first to fourteenth transistors is an n-channeltransistor.

Each display device in this embodiment mode corresponds to the displaydevices disclosed in this specification. Therefore, operation effectssimilar to those in the other embodiment modes are obtained.

This application is based on Japanese Patent Application serial No.2006-269689 filed in Japan Patent Office on Sep. 29, 2006, the entirecontents of which are hereby incorporated by reference.

1. A semiconductor device comprising: a first transistor; a secondtransistor; a circuit configured to apply an AC pulse to a gateelectrode of the second transistor; a clock wiring; a power supply line;and an output node, wherein the first transistor and the secondtransistor are connected in series between the clock wiring and thepower supply line, wherein the first transistor and the secondtransistor are connected to the output node, and wherein a semiconductorlayer including a channel formation region of each of the firsttransistor and the second transistor comprises an oxide semiconductor.2. The semiconductor device according to claim 1, wherein a capacitor isprovided between an electrode of the first transistor and the gateelectrode of the first transistor.
 3. The semiconductor device accordingto claim 1, wherein each of the first transistor and the secondtransistor is an n-channel transistor.
 4. The semiconductor deviceaccording to claim 1, wherein the oxide semiconductor is one of ZnO,a-InGaZnO, IZO, ITO, and SnO.
 5. An electronic device comprising thesemiconductor device according to claim
 1. 6. A semiconductor devicecomprising: a first transistor; a second transistor; a third transistor;a first clock wiring; a second clock wiring; a first power supply line;a second power supply line; and an output node, wherein the firsttransistor and the second transistor are connected in series between thefirst clock wiring and the first power supply line, wherein a firstelectrode of the third transistor is connected to a gate electrode ofthe second transistor, a second electrode of the third transistor isconnected to the second clock wiring, and a gate electrode of the thirdtransistor is connected to the second power supply line, wherein thefirst transistor and the second transistor are connected to the outputnode, and wherein a semiconductor layer including a channel formationregion of each of the first to third transistors comprises an oxidesemiconductor.
 7. The semiconductor device according to claim 6, whereina value of a ratio W/L of a channel width W to a channel length L of thefirst transistor is the greatest among values of ratios W/L of the firstto third transistors.
 8. The semiconductor device according to claim 6,wherein a capacitor is provided between the second electrode of thefirst transistor and the gate electrode of the first transistor.
 9. Thesemiconductor device according to claim 6, wherein each of the first tothird transistors is an n-channel transistor.
 10. The semiconductordevice according to claim 6, wherein the oxide semiconductor is one ofZnO, a-InGaZnO, IZO, ITO, and SnO.
 11. An electronic device comprisingthe semiconductor device according to claim
 6. 12. A display devicecomprising: a pixel portion including a plurality of pixels; and adriver circuit electrically connected to the pixel portion, wherein thedriver circuit includes a first transistor, a second transistor, a thirdtransistor, a fourth transistor, a fifth transistor, a sixth transistor,and a seventh transistor, wherein a first electrode of the firsttransistor is electrically connected to a fourth wiring, and a secondelectrode of the first transistor is electrically connected to a thirdwiring, wherein a first electrode of the second transistor iselectrically connected to a sixth wiring, and a second electrode of thesecond transistor is electrically connected to the third wiring, whereina first electrode of the third transistor is electrically connected to afifth wiring, a second electrode of the third transistor is electricallyconnected to a gate electrode of the second transistor, and a gateelectrode of the third transistor is electrically connected to a seventhwiring, wherein a first electrode of the fourth transistor iselectrically connected to the sixth wiring, a second electrode of thefourth transistor is electrically connected to the gate electrode of thesecond transistor, and a gate electrode of the fourth transistor iselectrically connected to a gate electrode of the first transistor,wherein a first electrode of the fifth transistor is electricallyconnected to the seventh wiring, a second electrode of the fifthtransistor is electrically connected to the gate electrode of the firsttransistor, and a gate electrode of the fifth transistor is electricallyconnected to a first wiring, wherein a first electrode of the sixthtransistor is electrically connected to the sixth wiring, a secondelectrode of the sixth transistor is electrically connected to the gateelectrode of the first transistor, and a gate electrode of the sixthtransistor is electrically connected to the gate electrode of the secondtransistor, and wherein a first electrode of the seventh transistor iselectrically connected to the sixth wiring, a second electrode of theseventh transistor is electrically connected to the gate electrode ofthe first transistor, and a gate electrode of the seventh transistor iselectrically connected to a second wiring, and wherein a semiconductorlayer including a channel formation region of each of the first toseventh transistors comprises an oxide semiconductor.
 13. The displaydevice according to claim 12, wherein a value of a ratio W/L of achannel width W to a channel length L of the first transistor is thegreatest among values of ratios W/L of the first to seventh transistors.14. The display device according to claim 12, wherein a value of a ratioW/L of a channel width W to a channel length L of the first transistoris two to five times greater than a value of a ratio W/L of the fifthtransistor.
 15. The display device according to claim 12, wherein achannel length L of the third transistor is larger than a channel lengthL of the fourth transistor.
 16. The display device according to claim12, wherein a capacitor is provided between the second electrode of thefirst transistor and the gate electrode of the first transistor.
 17. Thedisplay device according to claim 12, wherein each of the first toseventh transistors is an n-channel transistor.
 18. The display deviceaccording to claim 12, wherein the oxide semiconductor is one of ZnO,a-InGaZnO, IZO, ITO, and SnO.
 19. The display device according to claim12, wherein the driver circuit is a flip-flop circuit.
 20. An electronicdevice comprising the display device according to claim 12.